Add timezone‑naive handling for `expires_at`, switch to UTC, and validate resolver zone existence

• Convert timezone‑aware expires_at to naive datetime before storing in the database.
• Use datetime.utcnow() for all expiration checks instead of datetime.now().
• Verify a zone exists before creating a resolver; raise ResolverZoneNotFoundError if missing.
• Add the new ResolverZoneNotFoundError to error handlers and messaging.
• Simplify the device field in DeviceSmallResponse to DeviceSmallResponse | None.
• Remove the unused prompt="login" argument in the OIDC callback.
• Update imports and remove unused constants.