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

src/scribe/routes/snippets.py

@@ -205,6 +205,24 @@ async def update_snippet_route(snippet_id: int):
     return jsonify(out)
 
 
+@snippets_bp.route("/duplicates", methods=["GET"])
+@login_required
+async def duplicate_snippets_route():
+    """Near-duplicate snippets already recorded, grouped into merge candidates.
+
+    Registered ABOVE the `/<int:snippet_id>` routes on purpose — Quart matches
+    an int converter before a static segment either way, but keeping the literal
+    path first makes the precedence obvious to the next person reading this."""
+    uid = get_current_user_id()
+    try:
+        threshold = float(request.args.get("threshold", 0) or 0)
+    except (TypeError, ValueError):
+        threshold = 0.0
+    return jsonify(await dedup_svc.find_duplicate_snippets(
+        uid, threshold=threshold if threshold > 0 else None
+    ))
+
+
 @snippets_bp.route("/<int:snippet_id>/verify", methods=["POST"])
 @login_required
 async def verify_snippet_route(snippet_id: int):