feat: Implement basic caching layer with cache-aside pattern for /search results

- Implement cache-aside pattern via SearchCacheService in RouteSearch handler
- Add TTL policies: 3 hours near-term, 7 days far-term
- Write TestCacheAsideSearch and variants for TTL verification
- Extend CacheAside to fully implement Cache interface (Get, Set, Exists, Delete, Increment, Decrement)

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

@@ -89,11 +89,11 @@ Implement the complete multimodal trip planning service as specified in `docs/sp
 - [x] Write tests: TestRouteParetoRanking
 - [x] Run tests - must pass before task 7
 
-### Task 7: Basic caching layer [ ]
-- [ ] Implement cache-aside pattern for `/search` results
-- [ ] Add TTL policies: 2-6 hours for near-term dates, 7 days for far-term
-- [ ] Write tests: TestCacheAsideSearch
-- [ ] Run tests - must pass before task 8
+### Task 7: Basic caching layer [x]
+- [x] Implement cache-aside pattern for `/search` results
+- [x] Add TTL policies: 2-6 hours for near-term dates, 7 days for far-term
+- [x] Write tests: TestCacheAsideSearch
+- [x] Run tests - must pass before task 8
 
 ### Task 8: Station status endpoint [ ]
 - [ ] Implement `GET /v1/stations/{id}/status` endpoint