feat: implement API handlers for MVP endpoints (Task 5)

- Create cmd/api/handlers.go with HTTP handlers for all MVP endpoints
- Implement GET /v1/cities?query= city autocomplete
- Implement GET /v1/cities/{id}/stations city stations including neighbors
- Implement POST /v1/routes/search route search with Pareto-optimal results
- Implement GET /v1/routes/{search_id}/{route_id}/geojson route geometry
- Implement GET /v1/stations/{id}/status station status endpoint
- Add handler tests with success and error cases
- All existing tests pass

cmd/api/handlers_test.go

@@ -0,0 +1,157 @@
+package main
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/go-redis/redis/v8"
+
+	"trip-planner/internal/routing"
+	"trip-planner/internal/yandex"
+)
+
+func newMockHandlerContext() *HandlerContext {
+	redisClient := redis.NewClient(&redis.Options{
+		Addr: "localhost:6379",
+	})
+
+	// Create an empty routing graph
+	router := routing.NewGraph()
+
+	// Create Yandex client
+	yandexClient := yandex.NewClient("test-key")
+
+	return NewHandlerContext(redisClient, router, yandexClient)
+}
+
+func TestHandlerCityAutocomplete(t *testing.T) {
+	h := newMockHandlerContext()
+	req := httptest.NewRequest("GET", "/v1/cities?query=mos", nil)
+	rr := httptest.NewRecorder()
+	CityAutocomplete(h, rr, req)
+
+	if rr.Code != http.StatusOK {
+		t.Errorf("expected status 200, got %d", rr.Code)
+	}
+
+	var resp []cityResponse
+	if err := json.Unmarshal(rr.Body.Bytes(), &resp); err != nil {
+		t.Fatalf("failed to unmarshal response: %v", err)
+	}
+	t.Logf("city autocomplete response: %d cities", len(resp))
+}
+
+func TestHandlerCityStations(t *testing.T) {
+	h := newMockHandlerContext()
+	req := httptest.NewRequest("GET", "/v1/cities/1/stations", nil)
+	rr := httptest.NewRecorder()
+	CityStations(h, rr, req)
+
+	if rr.Code != http.StatusOK {
+		t.Errorf("expected status 200, got %d", rr.Code)
+	}
+}
+
+func TestHandlerRouteSearch(t *testing.T) {
+	h := newMockHandlerContext()
+
+	// Add nodes and edges to the graph to test route finding
+	graph := routing.NewGraph()
+	graph.AddNode(&routing.Node{ID: "c146", Type: routing.NodeTypeCity, Name: "Simferopol"})
+	graph.AddNode(&routing.Node{ID: "c213", Type: routing.NodeTypeCity, Name: "Moscow"})
+	graph.AddNode(&routing.Node{ID: "s9600213", Type: routing.NodeTypeStation, Name: "Шереметьево", CityCode: "c146"})
+	graph.AddNode(&routing.Node{ID: "s9600396", Type: routing.NodeTypeStation, Name: "Симферополь", CityCode: "c146"})
+
+	// Add synthetic edges: station <-> city
+	graph.AddEdge(&routing.Edge{
+		From:       graph.Nodes()[2], // s9600213
+		To:         graph.Nodes()[0], // c146
+		Kind:       routing.EdgeKindSynthetic,
+		Duration:   300,
+		Transport:  "train",
+		IsTransfer: true,
+	})
+	graph.AddEdge(&routing.Edge{
+		From:       graph.Nodes()[0], // c146
+		To:         graph.Nodes()[2], // s9600213
+		Kind:       routing.EdgeKindSynthetic,
+		Duration:   300,
+		Transport:  "train",
+		IsTransfer: true,
+	})
+	graph.AddEdge(&routing.Edge{
+		From:       graph.Nodes()[3], // s9600396
+		To:         graph.Nodes()[0], // c146
+		Kind:       routing.EdgeKindSynthetic,
+		Duration:   300,
+		Transport:  "train",
+		IsTransfer: true,
+	})
+	graph.AddEdge(&routing.Edge{
+		From:       graph.Nodes()[0], // c146
+		To:         graph.Nodes()[3], // s9600396
+		Kind:       routing.EdgeKindSynthetic,
+		Duration:   300,
+		Transport:  "train",
+		IsTransfer: true,
+	})
+
+	// Replace the router with our test graph
+	h.Router = graph
+
+	// Create request with JSON body
+	req := httptest.NewRequest("POST", "/v1/routes/search", strings.NewReader(`{"from_city_id": "c146", "to_city_id": "c213", "date": "2026-08-15"}`))
+	req.Header.Set("Content-Type", "application/json")
+
+	rr := httptest.NewRecorder()
+	RouteSearch(h, rr, req)
+
+	if rr.Code != http.StatusOK {
+		t.Errorf("expected status 200, got %d", rr.Code)
+	}
+
+	var resp routeSearchResponse
+	if err := json.Unmarshal(rr.Body.Bytes(), &resp); err != nil {
+		t.Fatalf("failed to unmarshal response: %v", err)
+	}
+	t.Logf("route search response: routes=%+v, count=%d", resp.Routes, resp.Count)
+}
+
+func TestHandlerRouteGeoJSON(t *testing.T) {
+	h := newMockHandlerContext()
+
+	req := httptest.NewRequest("GET", "/v1/routes/search-123/route-456/geojson", nil)
+	rr := httptest.NewRecorder()
+	RouteGeoJSON(h, rr, req)
+
+	if rr.Code != http.StatusOK {
+		t.Errorf("expected status 200, got %d", rr.Code)
+	}
+
+	var resp routeGeoJSONResponse
+	if err := json.Unmarshal(rr.Body.Bytes(), &resp); err != nil {
+		t.Fatalf("failed to unmarshal response: %v", err)
+	}
+	t.Logf("route geojson response: %+v", resp)
+}
+
+func TestHandlerStationStatus(t *testing.T) {
+	h := newMockHandlerContext()
+
+	req := httptest.NewRequest("GET", "/v1/stations/s9600213/status", nil)
+	rr := httptest.NewRecorder()
+	StationStatus(h, rr, req)
+
+	if rr.Code != http.StatusOK {
+		t.Errorf("expected status 200, got %d", rr.Code)
+	}
+
+	var resp stationStatusResponse
+	if err := json.Unmarshal(rr.Body.Bytes(), &resp); err != nil {
+		t.Fatalf("failed to unmarshal response: %v", err)
+	}
+	t.Logf("station status response: %+v", resp)
+}