feat(snippets): near-duplicate finder — surface the sets worth merging

#231's premise was unifying reusable things already scattered as one-offs.
The create gate PREVENTS a new duplicate and merge_snippets CURES one you
point it at, but nothing FOUND the duplicates already in the record —
someone had to notice them by hand, which is the exact failure the Drafter
exists to remove.

One indexed self-join over note_embeddings, not an N² Python scan:
pgvector's cosine distance is the same operator semantic search uses, so a
similarity floor is a distance ceiling and the work stays in Postgres.
`left.note_id < right.note_id` yields each unordered pair once and drops
the self-pair that would otherwise dominate the ranking.

Pairs are collapsed into merge SETS by connected components. Transitive on
purpose: A~B plus B~C puts all three together even when A and C don't
directly clear the bar, which is what merge actually does (it folds every
source into one survivor). The cost is that a chain of mild resemblances
can rope in a member that isn't really alike — so the UI presents a set as
a proposal, shows the members, and never merges without a confirm.

Two scope decisions worth naming:

- OWN snippets only. merge_snippets requires one owner across the set, so
  surfacing someone else's would propose a merge that cannot be performed.
  The report is bounded by what the operator can act on, not what they can
  see.

- Threshold defaults to 0.82, LOOSER than the write gate's 0.90, and is a
  setting rather than a constant (rule #25). The gate blocks a create and
  has to be unforgiving of noise; this only suggests a merge under review,
  so it must reach further or it would never surface the pairs the gate
  already let through — which are precisely the ones that accumulated.

Fixes a real bug in the merge flow while wiring the UI: selectedList
filtered the selection against the CURRENT PAGE, and doMerge derives its
source ids from that list. A corpus-wide suggested group with off-page
members would have rendered incomplete and silently merged only the
visible subset. A group under review is now the authority for that list.

Refs #2088

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

frontend/src/views/SnippetListView.vue

@@ -1,7 +1,13 @@
 <script setup lang="ts">
 import { ref, computed, onMounted } from "vue";
 import { useRouter } from "vue-router";
-import { listSnippets, mergeSnippets, type SnippetListItem } from "@/api/snippets";
+import {
+  findDuplicateSnippets,
+  listSnippets,
+  mergeSnippets,
+  type DuplicateGroup,
+  type SnippetListItem,
+} from "@/api/snippets";
 import { useToastStore } from "@/stores/toast";
 
 const router = useRouter();
@@ -50,13 +56,54 @@ const showMergeModal = ref(false);
 const canonicalId = ref<number | null>(null);
 const merging = ref(false);
 
-const selectedList = computed(() =>
-  snippets.value.filter((s) => selectedIds.value.has(s.id)),
-);
+// Near-duplicate report (#2088). Loaded on demand, not with the list: it's a
+// pairwise scan and most visits to this page aren't a tidy-up.
+const duplicateGroups = ref<DuplicateGroup[]>([]);
+const dupLoading = ref(false);
+const dupChecked = ref(false);
+// Set while merging a SUGGESTED group. The report reaches the whole corpus, so
+// its members need not all be on the current page — see selectedList.
+const reviewingGroup = ref<DuplicateGroup | null>(null);
+
+/** The records the merge modal acts on.
+ *
+ *  Normally that's the selection filtered against what's on screen. But a
+ *  suggested group is corpus-wide: filtering it by the current page would render
+ *  an incomplete set AND silently narrow what doMerge folds in, since it derives
+ *  its source ids from this list. When a group is under review it is the
+ *  authority. */
+const selectedList = computed<{ id: number; title: string }[]>(() => {
+  if (reviewingGroup.value) return reviewingGroup.value.snippets;
+  return snippets.value.filter((s) => selectedIds.value.has(s.id));
+});
+
+async function loadDuplicates() {
+  dupLoading.value = true;
+  try {
+    const data = await findDuplicateSnippets();
+    duplicateGroups.value = data.groups;
+    dupChecked.value = true;
+  } catch {
+    toast.show("Couldn't check for duplicates", "error");
+  } finally {
+    dupLoading.value = false;
+  }
+}
+
+/** Hand a suggested group to the existing merge flow, pre-selected. The operator
+ *  still picks which record survives and confirms — the report proposes, it
+ *  never merges. */
+function reviewGroup(group: DuplicateGroup) {
+  reviewingGroup.value = group;
+  selectedIds.value = new Set(group.note_ids);
+  canonicalId.value = group.note_ids[0] ?? null;
+  showMergeModal.value = true;
+}
 
 function exitSelectMode() {
   selectMode.value = false;
   selectedIds.value = new Set();
+  reviewingGroup.value = null;
 }
 function toggleSelectMode() {
   if (selectMode.value) exitSelectMode();
@@ -77,6 +124,15 @@ function openMerge() {
   canonicalId.value = selectedList.value[0]?.id ?? null;
   showMergeModal.value = true;
 }
+/** Dismiss the modal. Clears the reviewed group too — leaving it set would keep
+ *  selectedList pinned to a corpus-wide set the operator has walked away from. */
+function closeMerge() {
+  showMergeModal.value = false;
+  if (reviewingGroup.value) {
+    reviewingGroup.value = null;
+    selectedIds.value = new Set();
+  }
+}
 async function doMerge() {
   const target = canonicalId.value;
   if (target == null) return;
@@ -87,8 +143,13 @@ async function doMerge() {
     await mergeSnippets(target, sources);
     toast.show(`Merged ${sources.length} snippet${sources.length > 1 ? "s" : ""} in`);
     showMergeModal.value = false;
+    const wasSuggested = reviewingGroup.value !== null;
     exitSelectMode();
     await loadSnippets();
+    // The merged-away records are gone, so a stale report would keep offering
+    // them. Re-run it rather than clearing, so the operator can work through
+    // several groups without re-triggering the scan each time.
+    if (wasSuggested && dupChecked.value) await loadDuplicates();
   } catch {
     toast.show("Failed to merge snippets", "error");
   } finally {
@@ -259,6 +320,38 @@ function usageTitle(s: SnippetListItem): string {
       >
         {{ needsAttentionOnly ? "Needs attention · filtering" : "Needs attention" }}
       </button>
+      <button
+        class="btn-ghost"
+        :disabled="dupLoading"
+        title="Look for snippets already recorded that resemble each other closely enough to be worth merging"
+        @click="loadDuplicates"
+      >
+        {{ dupLoading ? "Checking…" : "Find duplicates" }}
+      </button>
+    </div>
+
+    <!-- Near-duplicate report. Only ever a proposal — merging is a separate,
+         confirmed act, and the operator chooses which record survives. -->
+    <div v-if="dupChecked && !dupLoading" class="dup-panel">
+      <p v-if="!duplicateGroups.length" class="dup-empty">
+        No near-duplicates found. Nothing recorded resembles anything else closely
+        enough to be worth merging.
+      </p>
+      <template v-else>
+        <p class="dup-head">
+          {{ duplicateGroups.length }} possible duplicate{{ duplicateGroups.length > 1 ? " sets" : " set" }}
+          — review each before merging; a set is a suggestion, not a verdict.
+        </p>
+        <div v-for="(g, i) in duplicateGroups" :key="i" class="dup-group">
+          <div class="dup-members">
+            <span v-for="s in g.snippets" :key="s.id" class="dup-member">
+              {{ splitTitle(s.title).name }}
+            </span>
+          </div>
+          <span class="dup-score">{{ Math.round(g.top_score * 100) }}% alike</span>
+          <button class="btn-ghost dup-action" @click="reviewGroup(g)">Review &amp; merge</button>
+        </div>
+      </template>
     </div>
 
     <!-- Reverse lookup: what's already kept in this repo / file / symbol. -->
@@ -389,7 +482,7 @@ function usageTitle(s: SnippetListItem): string {
 
     <!-- Merge modal -->
     <teleport to="body">
-      <div v-if="showMergeModal" class="modal-overlay" @click.self="showMergeModal = false">
+      <div v-if="showMergeModal" class="modal-overlay" @click.self="closeMerge">
         <div class="modal-card" role="dialog" aria-modal="true" aria-label="Merge snippets">
           <h3 class="modal-title">Merge snippets</h3>
           <p class="modal-desc">
@@ -409,7 +502,7 @@ function usageTitle(s: SnippetListItem): string {
             </label>
           </div>
           <div class="modal-actions">
-            <button class="modal-btn" @click="showMergeModal = false">Cancel</button>
+            <button class="modal-btn" @click="closeMerge">Cancel</button>
             <button
               class="modal-btn modal-btn-primary"
               :disabled="merging || canonicalId == null"
@@ -686,6 +779,63 @@ function usageTitle(s: SnippetListItem): string {
   color: var(--color-text-muted);
 }
 
+/* Near-duplicate report */
+.dup-panel {
+  margin-bottom: 1.25rem;
+  padding: 0.85rem 1rem;
+  border: 1px solid var(--color-border);
+  border-radius: 8px;
+  background: var(--color-surface-alt, var(--color-surface));
+}
+
+.dup-empty,
+.dup-head {
+  margin: 0 0 0.5rem;
+  font-size: 0.85rem;
+  color: var(--color-text-muted);
+}
+
+.dup-empty {
+  margin-bottom: 0;
+}
+
+.dup-group {
+  display: flex;
+  align-items: center;
+  gap: 0.75rem;
+  flex-wrap: wrap;
+  padding: 0.5rem 0;
+  border-top: 1px solid var(--color-border);
+}
+
+.dup-members {
+  display: flex;
+  gap: 0.4rem;
+  flex-wrap: wrap;
+  flex: 1 1 20rem;
+  min-width: 0;
+}
+
+.dup-member {
+  font-size: 0.8rem;
+  padding: 0.1rem 0.45rem;
+  border-radius: 4px;
+  background: color-mix(in srgb, var(--color-text-muted) 12%, transparent);
+  /* Long snippet names must not push the row into a horizontal scroll. */
+  overflow-wrap: anywhere;
+}
+
+.dup-score {
+  font-size: 0.75rem;
+  color: var(--color-text-muted);
+  font-variant-numeric: tabular-nums;
+  white-space: nowrap;
+}
+
+.dup-action {
+  white-space: nowrap;
+}
+
 /* Drift is a stronger signal than dead weight: the record may be actively
    misleading, not merely unused. Danger tone, and it sits first in the footer. */
 .drift-tag {