refactor(server): extract the artist-alias routes into routers/artist_aliases.py (R3) (#838)

* refactor(server): extract the artist-alias routes into routers/artist_aliases.py (R3)

Second router, picked by router_scan.py: the artist-aliases / Tidy-up (P4) group
ranks at 0 monkeypatch.setattr targets and 0 helpers to relocate. 5 routes
(list/set/merge/delete aliases + raw-artist picker), all meta_db-only.

Bodies verbatim; only @app.<m> -> @router.<m> and meta_db -> appstate.meta_db.
include_router mounts at the original site; full 143-route table identical to
origin/main (paths, methods, order). No test retargets: test_artist_alias drives
via TestClient(server.app) + server.meta_db, neither of which moved.

Verified: pyflakes clean on the router; no new undefined in server.py; JSONResponse
still used in server.py (not dead); pytest 2398 passed (18 in test_artist_alias);
packaging guard green; eslint 0; boot smoke drives all 5 routes end-to-end
(set ACDC->AC/DC, read back, 400 on missing fields, delete).

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

* docs: credit both router extractions in the size-exemptions rationale (CodeRabbit)

---------

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

CHANGELOG.md

@@ -26,6 +26,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   next root-level module can't ship broken.
 
 ### Added
+- **`routers/` — extracting `server.py`'s route layer, cheapest-first (R3).** Each PR moves a cohesive route group into a `fastapi.APIRouter` under `lib/routers/`, mounted with `app.include_router(...)` at its original site (FastAPI matches in registration order; the full route table stays byte-identical). Bodies are verbatim — only the decorator receiver (`@app` → `@router`) and singleton reads (`meta_db` → `appstate.meta_db`, resolved at call time) change. So far: `audio_effects` (5 routes), `artist_aliases` (5 routes — the Tidy-up/P4 alias overrides). The next cut is picked by a dependency-closure scan that ranks groups by how many `monkeypatch.setattr(server, …)` targets they'd drag along.
 - **`routers/` — the first extracted route module (R3).** The five audio-effects mapping
   endpoints move out of `server.py` into `lib/routers/audio_effects.py` as a
   `fastapi.APIRouter`, mounted with `app.include_router(...)` **at the point in the file