Introduces the ExtensionData namespace with the full contract and service
layer that future extension records (municipality profiles, election jobs,
service configs) will use to store and validate their legacy foreign references.

- LegacyLinkType enum (JurisdictionJCode, KitId, ContactId)
- LegacyLinkReference value record with typed factory methods
- ILegacyLinkValidator / LegacyLinkValidator — validates each link type
  against the anti-corruption layer; returns descriptive errors on failure
- ILegacyLinkedRecord / ILegacyLinkedRecordProvider — contracts extension
  record types implement to participate in integrity checks
- ILegacyLinkIntegrityCheck / LegacyLinkIntegrityService — scans all
  registered providers, computes consistency percentage (NFR13 target 99.9%)
- POST /api/admin/legacy-link/integrity-check (Admin-only, audit-logged)
- 19 unit tests covering all four ACs

Stories 1.10-1.13 register ILegacyLinkedRecordProvider implementations.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

- add Keycloak role mapping, app authorization policies, and logout/session coverage
- add shared append-only audit logging with authorization audit events
- add read-only legacy data access anti-corruption layer
- add legacy schema compatibility gate, admin API, and client schema check panel
- update story artifacts, planning docs, sprint status, and test coverage

- Add JWT bearer auth, session endpoint, and auth audit capture
- Add server-side Keycloak token exchange and refresh endpoints
- Load Keycloak client secret from ignored .env file
- Configure Canopy Keycloak realm, CORS, and Vite API proxy
- Add frontend protected auth callback, token storage, and refresh handling
- Cover backend and frontend auth behavior with tests