refactor(server): extract the chart routes into routers/chart.py (R3) (#846)

Unblocked by the DLC-path substrate (#843): chart's only server-module deps are
now app + meta_db (both seam); _get_dlc_dir/_resolve_dlc_path come from dlc_paths,
sloppak/loose detection from the shared lib modules. 4 routes (split/unsplit/
work/fileinfo), meta_db-only otherwise. 0 setattr targets, 0 helpers to relocate.

Bodies verbatim; @app -> @router, meta_db -> appstate.meta_db. include_router at
the original site; 143-route table identical to origin/main. No test retargeting.

server.py: 8,003 -> 7,909.

Verified: pyflakes clean on the router; no new undefined/dead in server.py; route
table identical; pytest 2401 passed (74 across work_charts/context_menu/
group_filter/packaging); eslint 0. Boot smoke: chart/work 200, chart/fileinfo
resolves the real pack path through _resolve_dlc_path.

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

docs/size-exemptions.md

@@ -55,8 +55,8 @@ without a *signed* exemption" is unenforceable.
 ## Planned, NOT exempt (owned by split plans — listed so nothing falls between states)
 
 core `static/app.js` (11,852) · `static/highway.js` (4,168, whole file) · `server.py`
-(8,003 — was 14,037; ratcheted by the R3 `MetadataDB` + `AudioEffectsMappingDB`
-extractions and five `routers/` modules) ·
+(7,909 — was 14,037; ratcheted by the R3 `MetadataDB` + `AudioEffectsMappingDB`
+extractions and six `routers/` modules) ·
 `lib/metadata_db.py` (4,373 — new in R3; the `MetadataDB` class alone is 4,018 lines
 and is a monolith in its own right, to be split per-table once the router train
 lands) · `static/v3/songs.js` (4,134) · `static/capabilities/audio-session.js`