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>

tests/test_playlists_api.py

@@ -6,6 +6,10 @@ import sys
 import pytest
 from fastapi.testclient import TestClient
 
+# Moved to routers/playlists in R3; reads appstate.config_dir, which the
+# `server` fixture configures via CONFIG_DIR before this is called.
+from routers.playlists import _playlist_cover_path
+
 
 @pytest.fixture()
 def server(tmp_path, monkeypatch, isolate_logging):
@@ -168,6 +172,6 @@ def test_cover_rejects_non_string_image_with_400_not_500(client):
 def test_deleting_playlist_removes_custom_cover(client, server):
     pid = client.post("/api/playlists", json={"name": "Doomed"}).json()["id"]
     client.post(f"/api/playlists/{pid}/cover", json={"image": _png_b64()})
-    assert server._playlist_cover_path(pid).exists()
+    assert _playlist_cover_path(pid).exists()
     client.delete(f"/api/playlists/{pid}")
-    assert not server._playlist_cover_path(pid).exists()
+    assert not _playlist_cover_path(pid).exists()