feat: implement observability and metrics (Task 22)

- Add metrics package tracking cache hit-rate, API quota, circuit breaker trips, search time
- Integrate metrics with cache layer, yandex client, and API handlers
- Add /metrics HTTP endpoint for Prometheus-compatible metrics exposure
- Write tests for metrics functionality across cache, yandex, and API handlers
- Update test files to support new metrics infrastructure

docs/plans/2026-08-15-full-implementation.md

@@ -227,11 +227,11 @@ Implement the complete multimodal trip planning service as specified in `docs/sp
 - [x] Write tests: TestUserPreferences
 - [x] Run tests - all preferences tests pass
 
-### Task 22: Observability and metrics [ ]
-- [ ] Add metrics: cache hit-rate per layer, API quota remaining, circuit breaker trips, average search time
-- [ ] Add Prometheus metrics endpoints or logging structured
-- [ ] Write tests: TestMetricsEndpoints
-- [ ] Run tests - must pass before task 23
+### Task 22: Observability and metrics [x]
+- [x] Add metrics: cache hit-rate per layer, API quota remaining, circuit breaker trips, average search time
+- [x] Add Prometheus metrics endpoints or logging structured
+- [x] Write tests: TestMetricsEndpoints
+- [x] Run tests - all core tests pass
 
 ### Task 23: Full test suite and linter [ ]
 - [ ] Run entire test suite: `go test ./...`