feat(library): metadata override + lock store, enforced by enrichment (popup slices 1–2)

Backend foundation for the Fix-metadata popup. Not yet surfaced in the UI (the
display + 3-tab popup are the next slices); no PR until it's user-visible.

Slice 1 — the store:
- `song_field_override(filename, field, value, locked)` table: a reversible
  DISPLAY overlay (never written to the pack), filename-keyed so it survives a
  rescan (never purged by delete_missing) and is dropped only with the song.
- DB methods (partial upsert that drops empty+unlocked rows; batch map) +
  `GET`/`PUT /api/song/{fn}/overrides` (field allowlist title/artist/album/
  year/genre; clearing rides PUT since DELETE /api/song/{path} shadows sub-
  routes; PUT demo-blocked).

Slice 2 — locks respected by enrichment:
- The auto-matcher composes a per-song `_compose_lock_filter` onto the global
  apply-filter, so a match still applies IDENTITY (mbid/release → art) but never
  re-canonicalizes a LOCKED display field.
- Gap-fill (write-to-file) skips locked album/year/genre — writing the matched
  value would be exactly the clobber the lock exists to prevent.
- Review/manual picks bypass the filter (an explicit confirm overrides a lock).

Tests: store semantics + rescan-survival + API; the lock filter + reader; an
auto-match leaving a locked field un-canonicalized; gap-fill excluding locked
keys.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_gap_fill.py

@@ -110,6 +110,19 @@ def test_preview_excludes_author_set_keys(server, client):
     assert {"genres", "mbid", "isrc"} <= got
 
 
+def test_preview_excludes_locked_fields(server, client):
+    """A field LOCKED in the Fix-metadata popup is never gap-filled — writing
+    the matched value would be exactly the clobber the lock prevents — even
+    though the match has a value and the manifest lacks it."""
+    make_dir_sloppak(server, "a.sloppak")
+    seed_match(server, "a.sloppak")
+    server.meta_db.set_song_override("a.sloppak", "album", locked=True)
+    server.meta_db.set_song_override("a.sloppak", "year", locked=True)
+    got = {m["key"] for m in client.get("/api/song/a.sloppak/gap-fill").json()["missing"]}
+    assert "album" not in got and "year" not in got
+    assert {"genres", "mbid", "isrc"} <= got        # unlocked keys still offered
+
+
 def test_preview_excludes_present_but_empty_keys(server, client):
     """Gap-fill is append-only, so a present-but-empty value (album: '',
     year: 0) is NOT a gap the writer can fill — appending would duplicate the