feat: PWA update strategy with hashed assets and cache control

Move frontend sources to frontend/, build content-hashed assets at deploy,
network-first SW for shell/JS, immutable cache for /assets/, version.json
update banner, and Cache-Control middleware. Docs in docs/PWA-STRATEGY.md.

Co-authored-by: Cursor <cursoragent@cursor.com>

server/app.py

@@ -20,6 +20,7 @@ from server.push import save_subscription, send_slot_reminder
 from server.scheduler import schedule_snooze, start_scheduler
 from server.settings import settings
 from server.slots import build_today
+from server.static_cache import CacheControlMiddleware
 from server.stats import build_history, check_milestones, compute_stats, MILESTONE_DEFS
 
 NO_STORE = {"Cache-Control": "no-store"}
@@ -63,6 +64,7 @@ async def lifespan(app: FastAPI):
 
 
 app = FastAPI(title="TakeYourMeds", version=settings.app_version, lifespan=lifespan)
+app.add_middleware(CacheControlMiddleware)
 
 
 @app.post("/api/auth/pin")