feat: add comprehensive health check endpoint

Author: drkaushiksarkar

.github/changes/feature_feat-9948.md

@@ -0,0 +1,12 @@
+# feat: add comprehensive health check endpoint
+
+## Summary
+- GET /health with dependency status checks
+- Database connectivity verification
+- Cache availability check
+- Upstream service health aggregation
+
+## Test plan
+- [x] Health check returns 200 when all deps healthy
+- [x] Returns 503 when critical dep is down
+- [x] Response time < 100ms unde