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

internal/routing/graph.go

@@ -162,8 +162,8 @@ func (g *Graph) buildAdjacencyList() map[string][]*Edge {
 	return adj
 }
 
-// nodesByID returns a node by its ID from the graph's nodes.
-func (g *Graph) nodesByID(id string) *Node {
+// NodesByID returns a node by its ID from the graph's nodes.
+func (g *Graph) NodesByID(id string) *Node {
 	for _, n := range g.nodes {
 		if n.ID == id {
 			return n
@@ -180,8 +180,8 @@ func (g *Graph) FindRoute(originID, destID string, opts SearchOptions) *Itinerar
 
 	// BFS with transfer tracking
 	// State: (nodeID, transfersUsed, accumulatedDuration, lastArrivalTime, path)
-	startNode := g.nodesByID(originID)
-	destNode := g.nodesByID(destID)
+	startNode := g.NodesByID(originID)
+	destNode := g.NodesByID(destID)
 
 	if startNode == nil || destNode == nil {
 		return nil
@@ -275,7 +275,7 @@ func (g *Graph) FindRoute(originID, destID string, opts SearchOptions) *Itinerar
 			// First leg: From is the origin node, subsequent legs use the previous edge's To
 			if len(current.itinerary.Legs) == 0 {
 				newLegs[len(current.itinerary.Legs)] = RouteLeg{
-					From:       g.nodesByID(originID), // origin node as From
+					From:       g.NodesByID(originID), // origin node as From
 					To:         nextNode,
 					Duration:   edge.Duration,
 					Transport:  edge.Transport,