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/cmd/server/main.go

@@ -70,6 +70,8 @@ func main() {
 	tagRuleRepo := postgres.NewTagRuleRepo(pool)
 	categoryRepo := postgres.NewCategoryRepo(pool)
 	poolRepo := postgres.NewPoolRepo(pool)
+	duplicatePairRepo := postgres.NewDuplicatePairRepo(pool)
+	dismissalRepo := postgres.NewDismissalRepo(pool)
 	transactor := postgres.NewTransactor(pool)
 
 	// Services
@@ -86,6 +88,9 @@ func main() {
 	tagSvc := service.NewTagService(tagRepo, tagRuleRepo, aclSvc, auditSvc, transactor)
 	categorySvc := service.NewCategoryService(categoryRepo, tagRepo, aclSvc, auditSvc)
 	poolSvc := service.NewPoolService(poolRepo, aclSvc, auditSvc)
+	duplicateSvc := service.NewDuplicateService(
+		fileRepo, duplicatePairRepo, dismissalRepo, aclSvc, auditSvc, transactor, cfg.DuplicateHashThreshold,
+	)
 	fileSvc := service.NewFileService(
 		fileRepo,
 		mimeRepo,
@@ -108,6 +113,7 @@ func main() {
 	authMiddleware := handler.NewAuthMiddleware(authSvc)
 	authHandler := handler.NewAuthHandler(authSvc)
 	fileHandler := handler.NewFileHandler(fileSvc, tagSvc, authSvc, cfg.MaxUploadBytes)
+	duplicateHandler := handler.NewDuplicateHandler(duplicateSvc)
 	tagHandler := handler.NewTagHandler(tagSvc, fileSvc)
 	categoryHandler := handler.NewCategoryHandler(categorySvc)
 	poolHandler := handler.NewPoolHandler(poolSvc)
@@ -117,7 +123,7 @@ func main() {
 
 	r, err := handler.NewRouter(
 		authMiddleware, authHandler,
-		fileHandler, tagHandler, categoryHandler, poolHandler,
+		fileHandler, duplicateHandler, tagHandler, categoryHandler, poolHandler,
 		userHandler, aclHandler, auditHandler,
 		cfg.StaticDir,
 		cfg.TrustedProxies,