feat: add version negotiated API compatibility gateway

[0xApana]

Implements issue #1025 by adding a version-negotiated API compatibility gateway for REST clients, the frontend, and the SDK.

Changes

• Added API contract negotiation using:
  • X-API-Version
  • Accept: application/vnd.bridge-watch.v1+json
• Added deterministic SHA-256 API contract fingerprints.
• Added consistent compatibility response headers:
  • X-API-Version
  • X-API-Contract
  • Vary: Accept, X-API-Version
  • Deprecation
  • Sunset
• Added compatibility discovery endpoints:
  • GET /api/v1/compatibility/versions
  • GET /api/v1/compatibility/contract
  • GET /api/v1/compatibility/capabilities
• Added 406 Not Acceptable responses for unsupported API versions.
• Added shared API contract fixtures for frontend and SDK consumers.
• Added SDK methods for contract, capability, and version discovery.
• Added frontend API helpers for compatibility inspection.
• Added dual-read and dual-write migration helpers for breaking domain changes.
• Documented pagination, error formats, timestamps, numeric precision, and migration windows.
• Added CI validation to detect missing or stale API contract fixtures.
• Added focused compatibility gateway tests.

Testing

Validated successfully:

• API contract fixture validation
• Compatibility module TypeScript checks
• SDK compatibility type checks
• ESLint checks for changed backend files
• Git diff formatting validation

Full repository builds and the complete test suite remain affected by existing unrelated dependency and type errors in the repository.

Closes #1025

[Mosas2000]

version-negotiated API compatibility gateway is well-designed and clean

[Mosas2000]

Nice work