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/SettingsView.vue

@@ -23,6 +23,10 @@ const kbInjectEnabled = ref(true);
 const kbInjectThreshold = ref("0.55");
 const kbInjectTopK = ref("3");
 const kbWritePathEnabled = ref(true);
+// Near-duplicate report floor. Deliberately looser than the 0.90 write-time
+// gate: that one BLOCKS a create and must be unforgiving of noise, this one only
+// suggests a merge the operator reviews (services/dedup.py).
+const kbDuplicateThreshold = ref("0.82");
 const savingKbInject = ref(false);
 const kbInjectSaved = ref(false);
 
@@ -68,8 +72,13 @@ async function saveRetention() {
 async function saveKbInject() {
   const t = Math.min(1, Math.max(0, Number(kbInjectThreshold.value) || 0));
   const k = Math.min(10, Math.max(1, Math.floor(Number(kbInjectTopK.value) || 1)));
+  // `|| 0.82` not `|| 0`: an unparseable value here should fall back to the
+  // default, not to 0 — a 0 floor would report every snippet as a duplicate of
+  // every other one.
+  const dupT = Math.min(1, Math.max(0, Number(kbDuplicateThreshold.value) || 0.82));
   kbInjectThreshold.value = String(t);
   kbInjectTopK.value = String(k);
+  kbDuplicateThreshold.value = String(dupT);
   savingKbInject.value = true;
   kbInjectSaved.value = false;
   try {
@@ -80,6 +89,7 @@ async function saveKbInject() {
       // Its own switch, but deliberately the same threshold/ceiling — see
       // WRITEPATH_ENABLED_KEY in services/plugin_context.py.
       kb_writepath_enabled: kbWritePathEnabled.value ? 'true' : 'false',
+      kb_duplicate_threshold: String(dupT),
     });
     kbInjectSaved.value = true;
     setTimeout(() => (kbInjectSaved.value = false), 2000);
@@ -464,6 +474,9 @@ onMounted(async () => {
     kbInjectTopK.value = allSettings.kb_autoinject_top_k;
   }
   kbWritePathEnabled.value = allSettings.kb_writepath_enabled !== "false";
+  if (allSettings.kb_duplicate_threshold !== undefined) {
+    kbDuplicateThreshold.value = allSettings.kb_duplicate_threshold;
+  }
   if (allSettings.notify_task_reminders !== undefined) {
     notifyTaskReminders.value = allSettings.notify_task_reminders !== "false";
   }
@@ -1211,6 +1224,25 @@ function formatUserDate(iso: string): string {
             edit. Off = prior art surfaces only on your own prompts.
           </p>
         </div>
+        <div class="field">
+          <label for="kb-duplicate-threshold">Near-duplicate report threshold</label>
+          <input
+            id="kb-duplicate-threshold"
+            v-model="kbDuplicateThreshold"
+            type="number"
+            min="0"
+            max="1"
+            step="0.01"
+            class="input"
+            style="max-width: 8rem"
+          />
+          <p class="field-hint">
+            How alike two snippets must be before the Snippets page suggests merging
+            them. Lower = more suggestions, more false pairs. Looser than the 0.90
+            used to block a duplicate at creation, because this only proposes a merge
+            you review — it never acts on its own.
+          </p>
+        </div>
         <div class="actions">
           <button class="btn-save" @click="saveKbInject" :disabled="savingKbInject">
             {{ savingKbInject ? 'Saving…' : 'Save' }}