feat: add feature flags ETag caching

[Baskarayelu]

Summary

• add strong ETags to GET /api/feature-flags using stable flag data
• honor If-None-Match with 304 responses while keeping request metadata out of the validator
• return 200 for stale tags and preserve injectable rate-limit behavior

Verification

• npx jest src/routes/feature-flags.test.ts --runInBand --forceExit (8 tests passed)

Note

The upstream route referenced a missing config.featureFlagsRateLimit; this PR falls back to the existing REST rate-limit configuration unless a route-specific middleware is injected.

Closes #1103