refactor(server): extract the settings routes into routers/settings.py (R3) (#863)

GET/POST /api/settings, /api/settings/reset, and the two-phase atomic
export/import bundle (/api/settings/export|import) move to lib/routers/settings.py
with their exclusive helpers (the relpath allowlist validator, the atomic writer,
the library-DB snapshot + sqlite integrity gate, the config-type validator, the
bundle schema). Bodies verbatim except @app->@router and the seam reads:
meta_db->appstate.meta_db, CONFIG_DIR->appstate.config_dir,
_running_version->appstate.running_version(), and _default_settings->
appstate.default_settings (the canonical defaults builder stays in server.py —
the scan + artist-links code share it — and is injected as a new seam callable).

server.py: 5,539 -> 4,478 (-1,061).

Verified: pyflakes clean (bar the pre-existing File/safe_join/tuning_name/ET);
route table IDENTICAL (143); full pytest 2397 passed (154 settings cases incl the
export→import round-trip + library-DB snapshot/restore + relpath-allowlist SSRF/
traversal guards, retargeted onto the settings module). eslint 0.

BEHAVIORAL — needs an on-device settings export→import round-trip sign-off before
merge (do not merge on green CI alone).

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

tests/test_feedpak_extension.py

@@ -14,6 +14,7 @@ back-compat for `.sloppak` libraries or stop accepting the new `.feedpak`:
 from __future__ import annotations
 
 import importlib
+from routers import settings as settings_router
 import io
 import sys
 import zipfile
@@ -242,7 +243,7 @@ def test_settings_dlc_count_includes_both_suffixes(tmp_path, settings_server):
     (dlc / "c.FEEDPAK").write_bytes(b"")   # case-insensitive (suffix.lower())
     (dlc / "notes.txt").write_bytes(b"")   # ignored
 
-    result = settings_server.save_settings({"dlc_dir": str(dlc)})
+    result = settings_router.save_settings({"dlc_dir": str(dlc)})
 
     assert "error" not in result, result
     # save_settings joins its notices into a single ``message`` string.