feat(static-assets): cache-bust JS/CSS via ?v=<commit-sha>, immutable in prod

Adds a `static_v(request, name, path=...)` Jinja helper that appends
?v=<commit-sha> from app.core.build_info, plus a CachedStaticFiles
subclass that serves Cache-Control: public, max-age=31536000, immutable
in production and no-cache in development. Browsers refetch JS/CSS
automatically on every deploy without the user having to hard-reload.

- New: app/core/static_files.py (CachedStaticFiles)
- Updated: app/templates_config.py (static_v helper)
- Updated: main.py (use CachedStaticFiles for *_static mounts)
- Codemod: 143 url_for('*_static', path='*.js'|'*.css') → static_v(...)
  across 123 templates. Images/fonts/JSON locales intentionally
  unchanged (out of scope).
- Arch rule: FE-024 (warning) flags raw url_for on JS/CSS to prevent
  drift. Note: FE-008 was already taken by the number_stepper rule.
- docs/proposals/static-asset-cache-busting.md marked Done.

Closes plan from docs/proposals/static-asset-cache-busting.md.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

main.py

@@ -40,6 +40,7 @@ from uvicorn.middleware.proxy_headers import ProxyHeadersMiddleware
 # ConnectionRefusedError at runtime. The isort: split below stops ruff from
 # alphabetically reordering this back behind api_router.
 from app.core import celery_config  # noqa: F401  # side-effect: set_default()
+from app.core.static_files import CachedStaticFiles
 
 # isort: split
 from app.api.main import api_router
@@ -235,12 +236,12 @@ if MODULES_DIR.exists():
         module_static = module_dir / "static"
         if module_static.exists():
             mount_path = f"/static/modules/{module_name}"
-            app.mount(mount_path, StaticFiles(directory=str(module_static)), name=f"{module_name}_static")
+            app.mount(mount_path, CachedStaticFiles(directory=str(module_static)), name=f"{module_name}_static")
             logger.info(f"Mounted module static files: {mount_path} -> {module_static}")
 
 # Mount main static directory AFTER module statics
 if STATIC_DIR.exists():
-    app.mount("/static", StaticFiles(directory=str(STATIC_DIR)), name="static")
+    app.mount("/static", CachedStaticFiles(directory=str(STATIC_DIR)), name="static")
     logger.info(f"Mounted static files from: {STATIC_DIR}")
 else:
     logger.warning(f"Static directory not found at {STATIC_DIR}")