Federate signer opinions by exact key ID

[jt55401]

Summary

• add canonical signer reputation, vote, and report routes keyed by exact key IDs
• store local and foreign opinions with idempotent vote semantics and migration-safe indexes
• preserve the documented legacy error envelope on the compatibility key-report route
• pin canonicalization 760593d4a02e9fffa56dc4d002eb52ab2ade1b49 and document clean-checkout operation

Validation

• npm test: 103 tests
• npm run openapi:lint
• npm run conformance: 12 fixtures plus the v1 smoke check
• git diff --check