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/internal/db/postgres/duplicate_repo.go

@@ -0,0 +1,145 @@
+package postgres
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+
+	"github.com/google/uuid"
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"tanabata/backend/internal/domain"
+	"tanabata/backend/internal/port"
+)
+
+// ---------------------------------------------------------------------------
+// DuplicatePairRepo
+// ---------------------------------------------------------------------------
+
+// DuplicatePairRepo implements port.DuplicatePairRepo using PostgreSQL.
+type DuplicatePairRepo struct {
+	pool *pgxpool.Pool
+}
+
+// NewDuplicatePairRepo creates a DuplicatePairRepo backed by pool.
+func NewDuplicatePairRepo(pool *pgxpool.Pool) *DuplicatePairRepo {
+	return &DuplicatePairRepo{pool: pool}
+}
+
+var _ port.DuplicatePairRepo = (*DuplicatePairRepo)(nil)
+
+// ReplaceAll atomically replaces the entire pairs table with the given set.
+// The rescan recomputes pairs from scratch, so a full DELETE + COPY is both
+// correct and the simplest way to drop pairs that no longer qualify.
+func (r *DuplicatePairRepo) ReplaceAll(ctx context.Context, pairs []domain.DuplicatePair) error {
+	tx, err := r.pool.Begin(ctx)
+	if err != nil {
+		return fmt.Errorf("DuplicatePairRepo.ReplaceAll begin: %w", err)
+	}
+	defer tx.Rollback(ctx) //nolint:errcheck // no-op after a successful commit
+
+	if _, err := tx.Exec(ctx, `DELETE FROM data.duplicate_pairs`); err != nil {
+		return fmt.Errorf("DuplicatePairRepo.ReplaceAll delete: %w", err)
+	}
+
+	if len(pairs) > 0 {
+		rows := make([][]any, len(pairs))
+		for i, p := range pairs {
+			rows[i] = []any{p.FileA, p.FileB, int16(p.Distance)}
+		}
+		if _, err := tx.CopyFrom(ctx,
+			pgx.Identifier{"data", "duplicate_pairs"},
+			[]string{"file_a", "file_b", "distance"},
+			pgx.CopyFromRows(rows),
+		); err != nil {
+			return fmt.Errorf("DuplicatePairRepo.ReplaceAll copy: %w", err)
+		}
+	}
+
+	if err := tx.Commit(ctx); err != nil {
+		return fmt.Errorf("DuplicatePairRepo.ReplaceAll commit: %w", err)
+	}
+	return nil
+}
+
+type pairRow struct {
+	FileA    uuid.UUID `db:"file_a"`
+	FileB    uuid.UUID `db:"file_b"`
+	Distance int16     `db:"distance"`
+}
+
+// ListVisible returns every stored pair where both files are live (not trashed),
+// the pair is not dismissed, and — for non-admins — both files are visible to the
+// viewer under the private-by-default model. This is the input to clustering.
+func (r *DuplicatePairRepo) ListVisible(ctx context.Context, viewerID int16, isAdmin bool) ([]domain.DuplicatePair, error) {
+	args := make([]any, 0, 4)
+	n := 1
+	aclWhere := ""
+	if !isAdmin {
+		var ca, cb string
+		ca, n, args = aclVisibilityCond("fa", objTypeFile, viewerID, n, args)
+		cb, n, args = aclVisibilityCond("fb", objTypeFile, viewerID, n, args)
+		aclWhere = "AND " + ca + " AND " + cb
+	}
+
+	sqlStr := fmt.Sprintf(`
+        SELECT p.file_a, p.file_b, p.distance
+        FROM data.duplicate_pairs p
+        JOIN data.files fa ON fa.id = p.file_a AND fa.is_deleted = false
+        JOIN data.files fb ON fb.id = p.file_b AND fb.is_deleted = false
+        WHERE NOT EXISTS (
+            SELECT 1 FROM data.duplicate_dismissals d
+            WHERE d.file_a = p.file_a AND d.file_b = p.file_b
+        )
+        %s
+        ORDER BY p.file_a, p.file_b`, aclWhere)
+
+	rows, err := r.pool.Query(ctx, sqlStr, args...)
+	if err != nil {
+		return nil, fmt.Errorf("DuplicatePairRepo.ListVisible: %w", err)
+	}
+	collected, err := pgx.CollectRows(rows, pgx.RowToStructByName[pairRow])
+	if err != nil {
+		return nil, fmt.Errorf("DuplicatePairRepo.ListVisible scan: %w", err)
+	}
+	out := make([]domain.DuplicatePair, len(collected))
+	for i, row := range collected {
+		out[i] = domain.DuplicatePair{FileA: row.FileA, FileB: row.FileB, Distance: int(row.Distance)}
+	}
+	return out, nil
+}
+
+// ---------------------------------------------------------------------------
+// DismissalRepo
+// ---------------------------------------------------------------------------
+
+// DismissalRepo implements port.DismissalRepo using PostgreSQL.
+type DismissalRepo struct {
+	pool *pgxpool.Pool
+}
+
+// NewDismissalRepo creates a DismissalRepo backed by pool.
+func NewDismissalRepo(pool *pgxpool.Pool) *DismissalRepo {
+	return &DismissalRepo{pool: pool}
+}
+
+var _ port.DismissalRepo = (*DismissalRepo)(nil)
+
+// Add records a pair as "not a duplicate". The two ids are stored in canonical
+// (file_a < file_b) order to match the table's CHECK and avoid (a,b)/(b,a)
+// duplicates; a repeated dismissal is a no-op.
+func (r *DismissalRepo) Add(ctx context.Context, a, b uuid.UUID, userID int16) error {
+	if bytes.Compare(a[:], b[:]) > 0 {
+		a, b = b, a
+	}
+	const sqlStr = `
+        INSERT INTO data.duplicate_dismissals (file_a, file_b, dismissed_by)
+        VALUES ($1, $2, $3)
+        ON CONFLICT (file_a, file_b) DO NOTHING`
+	q := connOrTx(ctx, r.pool)
+	if _, err := q.Exec(ctx, sqlStr, a, b, userID); err != nil {
+		return fmt.Errorf("DismissalRepo.Add: %w", err)
+	}
+	return nil
+}