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/config/config.go

@@ -63,6 +63,12 @@ type Config struct {
 	// Import
 	ImportPath string
 
+	// DuplicateHashThreshold is the maximum Hamming distance (out of 64) between
+	// two perceptual hashes for the files to be treated as duplicate candidates.
+	// Lower = stricter (fewer, more confident matches); higher = looser. Used only
+	// by the dedup rescan that (re)builds data.duplicate_pairs.
+	DuplicateHashThreshold int
+
 	// Static SPA. When set, the server serves the built frontend (and falls
 	// back to index.html for client routes) on the same port as the API. Empty
 	// in local development, where the Vite dev server serves the UI separately.
@@ -176,6 +182,8 @@ func Load() (*Config, error) {
 
 		ImportPath: requireStr("IMPORT_PATH"),
 
+		DuplicateHashThreshold: parseInt("DUPLICATE_HASH_THRESHOLD", 10),
+
 		StaticDir: defaultStr("STATIC_DIR", ""),
 	}