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/routers/admin.py

@@ -6,6 +6,7 @@ from app.dependencies import get_db, get_current_admin, bearer_scheme
 from app.models.user import User
 from app.schemas.user import AdminUserCreate, AdminPasswordReset, AdminUserResponse, Token
 from app.utils.security import hash_password, create_token
+from app.utils.notify import notify
 
 router = APIRouter(prefix="/api/admin", tags=["admin"])
 
@@ -73,6 +74,7 @@ async def disable_user(
 
     user.is_disabled = True
     await db.commit()
+    await notify("User disabled", f"Account disabled: {user.email} (by admin {current_admin.email})", priority="default")
 
 
 @router.post("/users/{user_id}/enable", status_code=status.HTTP_204_NO_CONTENT)
@@ -106,6 +108,7 @@ async def delete_user(
 
     await db.delete(user)
     await db.commit()
+    await notify("User deleted", f"Account deleted: {user.email} (by admin {current_admin.email})", priority="default")
 
 
 @router.post("/users/{user_id}/impersonate", response_model=Token)