refactor(server): move the metadata-enrichment subsystem into lib/enrichment.py (R3)

MusicBrainz / Cover-Art-Archive / AcoustID transport, the match-scorer glue, and
the background enrichment worker (~930 lines, 61 defs) leave server.py as one
cohesive unit. Bodies are verbatim; the only changes are seam reads:

  meta_db / config_dir / sloppak_cache_dir / art_cache_dir -> appstate.<slot>
  _song_pack_art_exists / _art_override_paths (stay in server.py for the art +
    delete routes) -> appstate.<callable> (new seam slots, injected by reference)
  _env_flag -> env_compat.env_flag_compat (the existing identical helper)
  _artist_title_from_filename -> imported from metadata_db (its home)
  the User-Agent VERSION lookup: Path(__file__).parent ->
    Path(__file__).resolve().parents[1] (lib/enrichment.py -> app root)

server.py drives the worker through the module (import enrichment; the routes +
scan lifecycle call enrichment.X). Tests that faked the network on `server`
(_mb_http_get, _enrich_network_enabled, _caa_http_get, ...) now patch the same
names on `enrichment` — the module attribute is resolved at call time, so one
setattr reaches both the routes and the worker's internal callers. Acyclic:
enrichment imports appstate/appconfig/dlc_paths/metadata_db/mb_match/
acoustid_match/sloppak/loosefolder, never server.

server.py: 6,917 -> 5,988 (-929).

Verified: pyflakes clean (bar the pre-existing File/safe_join/tuning_name/ET);
route table IDENTICAL (143); full pytest 2400 passed (140 enrichment/art cases
incl the offline-safety + transport-error-pauses-pass contracts that fake the
network); test_packaging 44 passed (enrichment.py resolves under lib/); eslint 0.
Boot smoke: /api/enrichment/status + POST /kick serve from the new module.

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

tests/test_field_overrides.py

@@ -6,6 +6,7 @@ song (delete_song). Locks pin a field against a later auto-match.
 """
 
 import importlib
+import enrichment
 import sys
 
 import pytest
@@ -149,7 +150,7 @@ def test_locked_fields_reader(server):
 
 
 def test_compose_lock_filter_strips_locked_cand_keys(server):
-    f = server._compose_lock_filter(None, {"artist", "year"})
+    f = enrichment._compose_lock_filter(None, {"artist", "year"})
     cand = {"recording_id": "r", "artist": "X", "artist_sort": "X", "title": "T",
             "year": "1990", "album": "A", "genres": ["rock"]}
     out = f(cand)
@@ -158,7 +159,7 @@ def test_compose_lock_filter_strips_locked_cand_keys(server):
     # …identity + unlocked display fields survive
     assert out["recording_id"] == "r" and out["title"] == "T" and out["album"] == "A"
     # no locks → base filter returned unchanged (zero-copy common path)
-    assert server._compose_lock_filter(None, set()) is None
+    assert enrichment._compose_lock_filter(None, set()) is None
 
 
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