Vendor django-q2 and add a throttled geocode queue

[Copilot]

• Review current geocode queue, site settings, and vendored django-q2 throttle behavior
• Run backend lint and the relevant backend tests to establish a baseline
• Add runtime geocode throttle profile/state models and migrations
• Snapshot provider identity onto queued geocode tasks and teach the broker to enforce runtime throttles at dequeue time
• Expose runtime throttle configuration and state through backend site settings/admin surfaces
• Update frontend site settings UI and types for runtime throttle controls
• Add or update backend tests for provider snapshotting, runtime throttle updates, and site settings behavior
• Re-run final validation and document the remaining unrelated frontend lint issue