refactor(server): extract the audio-effects routes into routers/audio_effects.py (R3) (#834)

The first route module through the appstate seam (#833). Picked BY MEASUREMENT,
not by the plan's guess: a transitive dep-closure scan over every route group
ranked audio-effects at 0 monkeypatch.setattr targets and exactly one exclusive
helper. (The same scan disproved the plan's assumption that artists/aliases was
free -- api_artist_links reaches _mb_http_get and _enrich_network_enabled, both
setattr targets.)

Bodies are verbatim. The only edits are mechanical:
  @app.get(...)            -> @router.get(...)
  audio_effect_mappings.x  -> appstate.audio_effect_mappings.x

The singleton read must stay a module attribute resolved at call time, so a
re-imported server re-publishes a fresh DB into the seam and monkeypatch reaches
this module. `routers/` never imports `server`: server -> routers -> appstate.

`app.include_router(...)` sits exactly where the routes used to be defined --
FastAPI matches in registration order, so the mount site preserves it. Verified
by diffing the FULL route table against origin/main: 143 routes, identical
paths, methods AND order.

server.py: 9,445 -> 9,386 lines. `fastapi.Query` went dead with the move and was
removed (the other four unused imports are pre-existing on main).

Packaging: COPY routers/ /app/routers/ plus `!routers/` + `!routers/**` in
.dockerignore (that file opens with a blanket `*`). Verified against the real
docker daemon: routers/ reaches the build context, __pycache__ does not.

Verified: pyflakes clean on routers/; no new undefined name in server.py;
pytest 2348 passed (75 in the audio-effects + demo-mode suites); eslint 0
errors; boot smoke drives all five routes end-to-end (create -> read back ->
activate -> clear -> delete -> 404 on missing -> 400 on bad body), Query(...)
still 422s on a missing required param, and demo mode still 403s all four
moved write routes while allowing the read.

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

routers/__init__.py

@@ -0,0 +1,22 @@
+"""FastAPI route modules extracted from ``server.py`` (R3).
+
+Each module here exposes a module-level ``router`` (a ``fastapi.APIRouter``)
+that ``server.py`` mounts with ``app.include_router(...)`` at the point in the
+file where those routes used to be defined — FastAPI matches routes in
+registration order, so keeping the mount site preserves it.
+
+**Routers must never ``import server``.** They reach core singletons through
+the injected seam instead::
+
+    import appstate
+
+    @router.get("/api/thing")
+    def get_thing():
+        return appstate.meta_db.thing()
+
+and always as a **module attribute, at call time** — never
+``from appstate import meta_db``, which freezes the binding and defeats both a
+later ``appstate.configure()`` and ``monkeypatch.setattr``. See ``appstate.py``.
+
+Dependencies flow one way: ``server -> routers -> appstate``.
+"""