feat(backend): perceptual hashing for images and video

Adds a 64-bit dHash perceptual hash (internal/imagehash, built on the existing
disintegration/imaging — no new dependency) and starts populating the long-unused
data.files.phash column:

- Upload sets phash inline for images (cheap, from the in-memory bytes).
- Replace recomputes it from new content for images and clears it for anything
  else, so a stale hash never survives a content swap.
- FileRepo.SetPHash sets/clears the hash (used by Replace and, later, the dedup
  backfill).
- DiskStorage.VideoFrameMiddle extracts a frame from the middle of a clip
  (ffprobe duration -> ffmpeg -ss duration/2), avoiding the shared-intro collision
  a fixed early offset causes. It is a concrete method, not part of the storage
  port: only the dedup CLI needs it, keeping ffmpeg off the upload path. Video
  phashes are therefore computed by that CLI, not at upload time.
- DUPLICATE_HASH_THRESHOLD config (default 10/64) for the later pair rescan.

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

backend/internal/db/postgres/file_repo.go

@@ -434,6 +434,18 @@ func (r *FileRepo) SetNeedsReview(ctx context.Context, ids []uuid.UUID, value bo
 	return nil
 }
 
+// SetPHash sets (or clears, when phash is nil) the perceptual hash of a file.
+// Used by the dedup backfill and on content replacement; phash is non-critical,
+// recomputable metadata, so callers may treat failures as best-effort.
+func (r *FileRepo) SetPHash(ctx context.Context, id uuid.UUID, phash *int64) error {
+	const sqlStr = `UPDATE data.files SET phash = $2 WHERE id = $1`
+	q := connOrTx(ctx, r.pool)
+	if _, err := q.Exec(ctx, sqlStr, id, phash); err != nil {
+		return fmt.Errorf("FileRepo.SetPHash: %w", err)
+	}
+	return nil
+}
+
 // ---------------------------------------------------------------------------
 // SoftDelete / Restore / DeletePermanent
 // ---------------------------------------------------------------------------