Add durable OGM thumbnail coverage pipeline

[ewlarson]

Summary

• add coverage reporting and cache priming for the exact gbl_resourceClass_sm=Maps result set, without an implicit provider or UNR filter
• generate and durably store OGM-owned thumbnails from metadata images, downloadable images, PDFs, COGs, and PMTiles, with local fallback visuals when source generation is unavailable
• refresh thumbnail state only when harvested source metadata changes
• enforce canonical access restrictions and bounded, SSRF-safe remote fetching
• document canary, backfill, verification, and rollback operations

Why

The OGM gallery had substantially lower thumbnail coverage than the comparable BTAA Maps gallery. Existing generation paths covered only a subset of source formats and could depend on ephemeral cache state.

This change gives OGM its own durable thumbnail pipeline and fallback coverage. Browser-facing image URLs remain OGM API URLs; this does not hotlink BTAA thumbnails.

Impact

Operators can measure and prime the same unfiltered Maps population shown by the gallery. Eligible public records receive either a source-derived OGM thumbnail or an OGM-hosted fallback, while restricted records remain excluded.

Validation

• targeted Python suite: 157 passed, 13 skipped
• final focused regression suite: 29 passed
• Ruff passed for all changed Python files
• git diff --check passed
• relevant modules passed Python compilation checks

The full database-backed integration suite was not rerun because the local primary ParadeDB volume had a stale lock; no production deployment or backfill is included in this PR.