feat: public feature flags endpoint

[David-282]

📝 Title

feat: public feature flags endpoint

Closes #183

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🟢 Summary

Exposes a public HTTP GET endpoint (/feature-flags) that returns active feature flag configurations for the calling client or user, drawing directly from the in-memory flag cache.

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🛠️ Changes Introduced

1. Public Route (src/routes/featureFlags.ts)

• Added GET /feature-flags handler utilizing getAllFlags() from src/services/featureFlags.ts.
• Formatted the response into a structured key-value map ({ [flagId]: { enabled, variant } }).
• Implemented structured logging using Pino (logger.info / logger.error).
• Handled correlation IDs by setting x-correlation-id response headers and including correlationId in response payloads.
• Applied standardized error envelope structure on internal failures.

2. Application Route Registration (src/routes/index.ts)

• Registered featureFlagsRouter under /feature-flags.

3. Unit Tests (tests/featureFlagsRoute.test.ts)

• Added Jest + Supertest unit tests covering:
  • Successful HTTP 200 response and correct flag mapping.
  • Header and body propagation of x-correlation-id.
• Maintained ≥ 90% test coverage on modified/added lines.

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✅ Acceptance Criteria Checklist

• Public endpoint returning flag values for the calling user/client implemented.
• Standardized response envelope and structured logging with correlation IDs included.
• Focused tests added and passing with ≥ 90% coverage.
• Followed repository linting and camelCase file naming conventions.

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🧪 How to Test Locally

1. Run Unit Tests & Check Coverage:

   npm run test -- tests/featureFlagsRoute.test.ts --coverage

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[greatest0fallt1me]

Clean implementation. LGTM 👍 merged.