Prepare OpenGeoMetadata API community transfer

[ewlarson]

Summary

• selectively port five compatible fixes from geobtaa/api, with provenance and exclusions recorded in docs/upstream_reconciliation.md
• reframe the README and public service identity around the draft ogm-mirror-network proposal while retaining the current 0.7.0 release line
• remove 2,108 generated static-map PNGs from the branch, ignore runtime/operator artifacts, and add an automated transfer-readiness gate
• add repository transfer, rehearsal, branch-inventory, and deployment-safety runbooks
• keep the current production Kamal service, image, hostname, database/index names, role topology, and bind-mount paths unchanged

Selected upstream fixes

• caf71d7: use Elasticsearch exists for similar-item checks
• 2d5444f: correct document-containment bounding-box behavior
• 78466a2: canonicalize dct:isSourceOf and clean relationship aliases/deduplication
• ac1b700: prime relationship caches through the service/database-pool path
• a51a018: isolate sitemap origins

This is intentionally not a wholesale merge to BTAA 0.8.11. The reconciliation ledger explains the compatibility boundary and the commits left out.

Verification

• full suite on an exact isolated sanitized candidate: 1,831 passed, 99 skipped, 1 xpassed
• make lint-check
• uv lock --check
• ./scripts/verify_transfer_readiness.sh
• isolated full-history rehearsal and strict git fsck

Production safety

config/deploy.yml is unchanged. Production remains pinned to the existing personal GHCR image path until a separately verified registry migration is deliberately scheduled. No Kamal deployment is performed by this PR.

Follow-up cutover work (not performed here)

• decide the disposition of three stale unmerged branches and delete only approved merged branches
• schedule a write freeze, run the rehearsed generated-artifact history rewrite, validate every rewritten ref, and force-push only with explicit authorization
• verify OpenGeoMetadata Actions policy/package access and recreate or confirm deployment secrets
• transfer the repository, update local/production remotes, run post-transfer checks, and separately plan any future GHCR ownership change

Draft status is intentional: the proposal language and transfer checklist should receive community/operations review before merge.