Add HSTS, gzip, Ntfy notifications, and paginate entries endpoint

- nginx: add HSTS (max-age=31536000) and gzip for text/css/js/json
- entries: add limit/offset query params (default 200, max 1000)
- Ntfy: wire NTFY_URL/NTFY_TOKEN through env; notify on startup, new
  registration, admin login, user disable, and user delete
- httpx==0.28.1 added for async Ntfy HTTP calls

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/main.py

@@ -43,6 +43,8 @@ async def lifespan(app: FastAPI):
     await init_db()
     await _seed_admin()
     logger.info("Bourbonacci started")
+    from app.utils.notify import notify
+    await notify("Bourbonacci started", "Service startup complete.", priority="low")
     yield
     logger.info("Bourbonacci shutting down")