feat(routes): expose bounded retained report evidence

[n30nex]

Saved routes gain bounded retained-report evidence through GET /api/v1/routes/{iata}/{pathKey}/observations. Matching uses the entire stored prefix path, width and IATA, with cursor-bound time windows and pagination. It does not equate equal short prefixes with confirmed physical identities, and expired/unclassified/TRACE evidence is excluded. Migration 041 supplies the supporting index.

Closes #171. Follows server #169. Parent 39c9d185087dcf6bc5aede8f8d1e86c61c1365c9, head 0fc1398eecaa24f1dbdd0fae28bc259f15a439f2. Focused diff.

Validated after integration with accepted #177–#180: local build/format/vet/tests and published-head CI pass. The composed Pi build 2ed2e031 includes all current server candidates and independent #166; its full PostgreSQL suite passes, including route-lock batching/cancellation and queue behavior. An isolated 504-scope replay preserved 100 packets, 800 observations, 100 decrypted messages and 2,400 opt-in live hearings from 3,200 inputs while a later maintenance batch was blocked. This is fixture evidence, not a universal delivery guarantee.

The backend is deployed on the Pi preview with exact source and changelog, retaining configuration, schema042 and rollback. All earlier review features remain included. Owners retain merges and production release. AI-assisted implementation continues under the contributor’s standing project authorization.

[n30nex]

@MrAlders0n, ready for your / Claude's review at f473b165. The declared server stack has been refreshed for accepted #170; this PR body now links its current focused diff and dependency. Full native Pi/PostgreSQL tests pass in composed server 99e623c5, with every existing candidate included. Exact-head CI passes.

The preview/source now runs server 99e623c5 / web 3b3abdcc. Migration 041 was tested on a restored copy with unchanged row fingerprints; prior database/binary/web recovery is retained. Server merge order is #167 -> #169 -> #172; #166 remains independent. No upstream merge or production switch was performed.