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/config.py

@@ -1,3 +1,4 @@
+from typing import Optional
 from pydantic_settings import BaseSettings
 
 
@@ -9,6 +10,8 @@ class Settings(BaseSettings):
     admin_username: str
     admin_password: str
     allowed_origins: str = "http://localhost"
+    ntfy_url: Optional[str] = None
+    ntfy_token: Optional[str] = None
 
     class Config:
         env_file = ".env"