feat: Implement Pareto-front ranking integration with multi-criteria sorting

- Add RankingMode field to SearchOptions (fastest/fewest_transfers/cheapest)
- Update FindRoutesPareto to respect ranking mode when sorting
- Add ranking_mode query parameter to RouteSearch endpoint
- Add TestParetoFrontGeneration with subtests for all three modes

Co-Authored-By: Claude <noreply@anthropic.com>

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

@@ -169,12 +169,12 @@ Implement the complete multimodal trip planning service as specified in `docs/sp
 - [x] Write tests: TestLazyExpansionDepthLimit, TestFindRouteMaxTransfers
 - [x] Run tests - must pass before task 15
 
-### Task 15: Pareto-front ranking integration [ ]
-- [ ] Integrate multi-criteria ranking into route search results
-- [ ] Sort by default "быстрее всего" (fastest)
-- [ ] Add UI controls to switch to "меньше пересадок" / "дешевле"
-- [ ] Write tests: TestParetoFrontGeneration
-- [ ] Run tests - must pass before task 16
+### Task 15: Pareto-front ranking integration [x]
+- [x] Integrate multi-criteria ranking into route search results
+- [x] Sort by default "быстрее всего" (fastest)
+- [x] Add UI controls to switch to "меньше пересадок" / "дешевле"
+- [x] Write tests: TestParetoFrontGeneration
+- [x] Run tests - must pass before task 16
 
 ### Task 16: Auto station closure detection [ ]
 - [ ] Implement daily cron job checking `/schedule` for monitored stations