- wire legacy schema checks to live OleDb inspection and durable history
- harden schema baseline parsing and add constraint drift detection
- authenticate admin schema API calls from the client
- add release-gate workflow for legacy schema validation
- add extension-record save path with pre-save legacy link validation
- add link ambiguity detection, provider coverage reporting, and nightly integrity scheduler
- mark stories 1.7 and 1.8 done after review fixes

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>