refactor(server): extract MetadataDB into lib/metadata_db.py (R3) (#830)

Move-only. The library metadata cache -- the `MetadataDB` class (4,018 lines)
plus the query helpers it owns (keyset paging cursors, the tuning grouping key,
smart-arrangement naming, tag normalisation, the startup DB-restore swap) --
moves out of server.py into a flat `lib/` module. Every moved block is
byte-identical to its server.py original; server.py is exactly origin/main
minus the six cut ranges, minus the now-dead `import contextlib`, plus the
import-back block and the constructor call site.

server.py: 14,037 -> 9,705 lines.

The one non-verbatim change is the seam that lets the class leave server.py:
`MetadataDB.__init__` now takes `config_dir` explicitly instead of reading the
module-level CONFIG_DIR, so `lib/metadata_db.py` does no IO at import
(Principle V). The `meta_db` singleton stays in server.py, so `server.meta_db`
(282 refs) and `server.app` (67 refs) resolve unchanged and no route moves.
None of the 114 `monkeypatch.setattr(server, ...)` targets moved.

Logging still goes through the `feedBack.server` logger, so log filters and
caplog assertions resolve to the same logger object.

`tests/test_settings_export_library_db.py` imports `_apply_pending_db_restore`
from metadata_db (the test moves with its subject); no other test changed.

Verified: pyflakes clean on the new module (zero undefined names, zero unused
imports) and no new undefined name in server.py; pytest 2341 passed;
node --test 1030 passed; eslint 0 errors; uvicorn boot smoke serves
/api/version, /api/library, and all three migrated plugins' src/ module graphs
(stems, studio, editor -> 200).

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

tests/test_settings_export_library_db.py

@@ -22,6 +22,11 @@ from pathlib import Path
 import pytest
 from fastapi.testclient import TestClient
 
+# The startup DB swap lives with the DB layer it guards, not with server.py.
+# metadata_db reads no environment at import, so a plain module import is safe
+# alongside the env-patched `server_mod` fixture below.
+from metadata_db import _apply_pending_db_restore
+
 
 @pytest.fixture()
 def server_mod(tmp_path, monkeypatch):
@@ -219,7 +224,7 @@ def test_apply_pending_db_restore_swaps_and_clears_sidecars(server_mod, tmp_path
     (tmp_path / "web_library.db-shm").write_bytes(b"OLD-SHM")
     (tmp_path / "web_library.db.restore").write_bytes(new_db)
 
-    server_mod._apply_pending_db_restore(tmp_path)
+    _apply_pending_db_restore(tmp_path)
 
     assert main.read_bytes() == new_db                  # swapped in
     assert not (tmp_path / "web_library.db.restore").exists()
@@ -234,7 +239,7 @@ def test_apply_pending_db_restore_discards_corrupt_keeps_live(server_mod, tmp_pa
     main.write_bytes(b"LIVE-GOOD-DB")
     (tmp_path / "web_library.db.restore").write_bytes(b"SQLite format 3\x00" + b"\xff" * 64)
 
-    server_mod._apply_pending_db_restore(tmp_path)
+    _apply_pending_db_restore(tmp_path)
 
     assert main.read_bytes() == b"LIVE-GOOD-DB"          # live DB preserved
     assert not (tmp_path / "web_library.db.restore").exists()  # bad restore dropped
@@ -242,7 +247,7 @@ def test_apply_pending_db_restore_discards_corrupt_keeps_live(server_mod, tmp_pa
 
 def test_apply_pending_db_restore_noop_without_staging(server_mod, tmp_path):
     (tmp_path / "web_library.db").write_bytes(b"LIVE")
-    server_mod._apply_pending_db_restore(tmp_path)        # nothing staged
+    _apply_pending_db_restore(tmp_path)        # nothing staged
     assert (tmp_path / "web_library.db").read_bytes() == b"LIVE"
 
 
@@ -266,7 +271,7 @@ def test_full_db_backup_restore_round_trip(client, server_mod, tmp_path):
     # Simulate a restart: close the live conn, apply the staged restore,
     # reopen — the song is back.
     server_mod.meta_db.conn.close()
-    server_mod._apply_pending_db_restore(tmp_path)
+    _apply_pending_db_restore(tmp_path)
     conn = sqlite3.connect(str(tmp_path / "web_library.db"))
     try:
         rows = conn.execute(