refactor(server): extract the playlists routes into routers/playlists.py (R3) (#841)

The biggest router yet — 12 playlist routes + custom covers — and the first that
needs the config-path seam. server.py: 9,302 -> 9,085 (-217).

Three causally-linked pieces, all required for playlists:
- `config_dir` joins the appstate seam (the plan always put the path constants
  there; deferred in S3, needed now). It's env-derived, so the ~49
  pop-and-reimport fixtures reconfigure it for free — ZERO setattr retargeting.
  STATIC_DIR/SLOPPAK_CACHE_DIR (patched via setattr) stay in server.py until a
  router that reads them is extracted, and get retargeted then.
- `_clean_str` (pure request-field sanitizer, 14 callers) -> lib/reqfields.py;
  server.py imports it back. Unblocks wanted/saved/collections/profile/... later.
- routers/playlists.py: bodies verbatim, `@app`->`@router`, `meta_db`->
  `appstate.meta_db`, `CONFIG_DIR`->`appstate.config_dir`, `_clean_str` from
  reqfields, `_ART_CACHE_HEADERS` as a local const (art keeps server.py's).
  The two exclusive cover helpers (_playlist_cover_path/_url) move with it.

include_router at the original site; full 143-route table identical to
origin/main. One test retarget: test_playlists_api called
`server._playlist_cover_path` directly -> now imports it from routers.playlists
(reads appstate.config_dir, which the `server` fixture configures).

Verified: pyflakes clean; route table identical; pytest 2401 passed (28 in
playlists+collections+appstate); packaging guard 51 (auto-picked up reqfields);
eslint 0; boot smoke drives create/rename/add-song/cover-upload/serve/delete —
the cover writes 1.png under CONFIG_DIR THROUGH appstate.config_dir and serves
200 with an mtime cache-bust token; a wrong-typed name field still 400s via
_clean_str; demo untouched.

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

CHANGELOG.md

@@ -26,7 +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), `artist_aliases` (5), `loops` (3 — saved A/B practice regions). 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/` — 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), `artist_aliases` (5), `loops` (3), `playlists` (12 + custom covers). The `config_dir` path constant now rides the `appstate` seam (env-derived, so the pop-and-reimport fixtures reconfigure it for free), and the shared request-field sanitizer `_clean_str` moved to `lib/reqfields.py`. 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