refactor(server): move the metadata-enrichment subsystem into lib/enrichment.py (R3) (#861)

* 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>

* test: reset enrichment worker state between tests (CodeRabbit)

lib/enrichment.py now owns the worker, and it stays imported for the whole
session while the `server` fixtures pop-and-reimport `server` — so the cancel
Event / status dict / caches would leak across tests, and a stale `_enrich_cancel`
could short-circuit a later direct `_background_enrich()`. An autouse conftest
fixture clears that process-global state before each test.

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

* test: tighten the enrichment-reset fixture (CodeRabbit)

Narrow the import guard to ImportError (not blind Exception, BLE001), and stop
clearing _caa_index_locks — it's guarded by _caa_index_locks_guard, so an
unlocked clear() would race a still-alive worker, and its per-release mutexes
carry no test state.

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

---------

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

docs/size-exemptions.md

@@ -55,8 +55,8 @@ without a *signed* exemption" is unenforceable.
 ## Planned, NOT exempt (owned by split plans — listed so nothing falls between states)
 
 core `static/app.js` (11,852) · `static/highway.js` (4,168, whole file) · `server.py`
-(6,917 — was 14,037; ratcheted by the R3 `MetadataDB` + `AudioEffectsMappingDB`
-extractions and fifteen `routers/` modules) ·
+(5,988 — was 14,037; ratcheted by the R3 `MetadataDB` + `AudioEffectsMappingDB`
+extractions and fifteen `routers/` modules; the ~930-line metadata-enrichment subsystem — MB/CAA/AcoustID transport, matcher, background worker — now lives in `lib/enrichment.py`) ·
 `lib/metadata_db.py` (4,373 — new in R3; the `MetadataDB` class alone is 4,018 lines
 and is a monolith in its own right, to be split per-table once the router train
 lands) · `static/v3/songs.js` (4,134) · `static/capabilities/audio-session.js`