[codex] Move OGM backend changes into parent app

[ewlarson]

Summary

This draft PR moves the backend changes that were accidentally made in the child ogm-api repo into the parent geobtaa/api backend app.

It includes:

• durable-first thumbnail cache priming behavior
• public OGM repository dashboard at /api/v1/ogm/repos/dashboard
• richer public OGM repo summary fields for GitHub metadata, harvest freshness, and API availability counts
• nightly OGM repo discovery + harvest trigger script
• GitHub Actions workflow to run the nightly sync on production over SSH
• scheduled nightly watch-mode defaults while retaining existing weekly, webhook, both, and manual support
• separate throttling bucket for analytics event ingestion and CORS preflight bypass behavior
• docs and tests for the migrated OGM/rate-limit behavior

Notes

The Nominatim implementation from the child repo was already present in geobtaa/api, so this PR does not copy that service over. I also avoided wholesale file replacement where the parent app had newer local changes.

Validation

Passed locally from the parent checkout:

DATABASE_URL=postgresql+asyncpg://postgres:postgres@localhost:5432/testdb \
DB_HOST=localhost \
DB_PORT=5432 \
TEST_DB_NAME=testdb \
python -m pytest \
  tests/middleware/test_rate_limit_middleware.py \
  tests/middleware/test_rate_limit_integration.py \
  tests/tasks/test_ogm_harvest_tasks.py \
  tests/api/v1/test_ogm_public_endpoints.py

Result: 36 passed in 0.79s.

Also passed:

git diff --check
python -m compileall backend/app/api/v1/endpoint_modules/ogm.py backend/app/middleware/rate_limit_middleware.py backend/app/services/ogm_harvest/repository.py backend/app/tasks/ogm_harvest.py backend/scripts/populate_ogm_repos.py backend/scripts/trigger_ogm_nightly_sync.py