feat(backend): duplicate pairs, dismissals, and merge resolution

Adds the duplicate-detection backend on top of perceptual hashing:

- Two tables (edited into the original migrations): data.duplicate_pairs holds
  precomputed near-duplicate candidates (rebuilt wholesale by the rescan), and
  data.duplicate_dismissals is a global "not a duplicate" overlay that survives
  rescans. New audit actions file_merge / duplicate_dismiss.
- DuplicateService:
  - Rescan builds every pair within DUPLICATE_HASH_THRESHOLD via a BK-tree over
    the perceptual hashes and replaces the pairs table. This is the only thing
    that populates pairs, so GET never compares all-vs-all (scales to 110k+).
  - Clusters reads the precomputed pairs (ACL-filtered, non-trashed, non-
    dismissed), groups them into connected components via union-find, and
    paginates whole clusters.
  - Resolve merges a pair field-by-field: each scalar from keep or discard,
    metadata keep/discard/shallow-merge, tags/pools keep or union; then trashes
    the discarded file. Enforces edit ACL on both.
  - Dismiss records a canonical pair (view ACL on both).
- Endpoints under /files: GET /files/duplicates, POST /files/duplicates/dismiss,
  POST /files/duplicates/resolve (registered before /:id to avoid collision).
  Plain delete reuses /files/bulk/delete.
- Repo support: ListMissingPHash, ListAllPHashes, CopyPoolMemberships, plus the
  DuplicatePairRepo (ReplaceAll via COPY, ListVisible) and DismissalRepo.

Unit tests cover the BK-tree pairing, union-find clustering, metadata merge and
field validation; an integration test covers rescan -> list -> merge -> dismiss
(including that a dismissal survives a re-rescan).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/migrations/006_indexes.sql

@@ -26,6 +26,12 @@ CREATE INDEX idx__files__needs_review      ON data.files USING btree (id) WHERE
 CREATE INDEX idx__file_tag__tag_id  ON data.file_tag USING hash (tag_id);
 CREATE INDEX idx__file_tag__file_id ON data.file_tag USING hash (file_id);
 
+-- data.duplicate_pairs / data.duplicate_dismissals
+-- The composite primary keys cover lookups on file_a; these add the file_b side
+-- (used by the ON DELETE CASCADE and by the visibility join on the second file).
+CREATE INDEX idx__duplicate_pairs__file_b      ON data.duplicate_pairs      USING hash (file_b);
+CREATE INDEX idx__duplicate_dismissals__file_b ON data.duplicate_dismissals USING hash (file_b);
+
 -- data.pools
 CREATE INDEX idx__pools__creator_id ON data.pools USING hash (creator_id);
 
@@ -70,6 +76,8 @@ DROP INDEX IF EXISTS activity.idx__sessions__token_hash;
 DROP INDEX IF EXISTS activity.idx__sessions__user_id;
 DROP INDEX IF EXISTS acl.idx__acl__user;
 DROP INDEX IF EXISTS acl.idx__acl__object;
+DROP INDEX IF EXISTS data.idx__duplicate_dismissals__file_b;
+DROP INDEX IF EXISTS data.idx__duplicate_pairs__file_b;
 DROP INDEX IF EXISTS data.idx__file_pool__file_id;
 DROP INDEX IF EXISTS data.idx__file_pool__pool_id;
 DROP INDEX IF EXISTS data.idx__pools__creator_id;