Add structured observability foundation

[OkeyAmy]

Summary

• Provide a safe, structured observability foundation so requests can be traced (correlation ids), high-level metrics are captured, and sensitive data is never leaked in logs.
• Instrument core API and DB connection points to measure latency, error counts, and database query shapes for development without exposing secrets.

Description

• Add a JSON-lines logger with recursive redaction of sensitive keys and bounded error serialization in lib/observability/logger.ts and a small LogContext shape.
• Add an in-process low-cardinality metrics collector in lib/observability/metrics.ts and use it to count requests/errors and record latencies.
• Instrument the shared route wrapper in lib/api/route-handler.ts to record request duration, increment http.requests/http.errors counters, record http.duration, and emit structured completion logs with X-Correlation-Id.
• Instrument lib/dbConnect.ts to measure DB connect latency, record DB operation counters, log connection failures, and enable a development-only mongoose debug hook that emits query shapes (no values).
• Add a safe readiness/liveness endpoint at GET /api/health that returns X-Correlation-Id and omits configuration/secrets (app/api/health/route.ts).
• Add documentation for the observability conventions in docs/observability.md.
• Add unit tests for redaction/error serialization (lib/observability/logger.test.ts) and the health endpoint readiness/secret-omission behavior (app/api/health/route.test.ts).

Testing

• Ran unit tests: npm test -- --run lib/observability/logger.test.ts app/api/health/route.test.ts, results: 2 logger tests + 1 health test passed (3 tests total, all green).
• Static checks: npm run typecheck completed without errors.
• Linting: npm run lint completed without errors.
• Git/diff validation: git diff --check returned clean.

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Close #79 , #80 , #84

[OkeyAmy]

#80

[OkeyAmy]

#84