feat: lazy graph expansion - hub station selection, on-demand /search, and transfer depth limiting

- Remove Population field from HubStation; hub selection now uses only outgoing flights criterion
- Simplify SelectHubStations criteria (minPopulation removed from function calls)
- Add synthetic edge fallback in FindRoute when lazy expansion fails
- Add ResetCircuitBreaker helper to yandex client for test reset
- Update test criteria to match new hub selection logic
- Remove TestLazySearchCacheIntegration (replaced by integration tests)

internal/routing/graph.go

@@ -53,8 +53,8 @@ const (
 
 // hubCriteria defines the criteria for selecting hub stations.
 type hubCriteria struct {
-	minPopulation        int      // minimum city population in millions to be considered a hub
 	minOutgoingFlights   int      // minimum number of outgoing Yandex flights to be considered a hub
+	minPopulation        int      // minimum city population (millions) to be considered a hub
 	defaultOutgoingFlights int     // default outgoing flights count when data is unavailable
 }
 
@@ -66,8 +66,6 @@ type HubStation struct {
 	CityCode string
 	// OutgoingFlights is the estimated number of outgoing Yandex flights from this station.
 	OutgoingFlights int
-	// Population is the city population in millions used for hub selection.
-	Population int
 	// IsHub indicates whether this station meets the hub criteria.
 	IsHub bool
 }
@@ -93,13 +91,12 @@ func SelectHubStations(stations []StationInfo, criteria hubCriteria) HubStationS
 			Station:        &Node{ID: si.ID, Type: NodeTypeStation, Name: si.Name, CityCode: si.CityCode},
 			CityCode:       si.CityCode,
 			OutgoingFlights: criteria.defaultOutgoingFlights,
-			Population:     0, // will be inferred from city code later
 			IsHub:          false,
 		}
 
-		// A station is considered a hub if:
-		// 1. It has >= minOutgoingFlights (outgoing Yandex flight data available) - primary criterion
-		// For MVP, outgoing flights is the primary criterion.
+		// A station is considered a hub if it has >= minOutgoingFlights outgoing Yandex flights.
+		// Population-based selection (minPopulation) is tracked for future implementation;
+		// currently only the outgoing flights criterion is enforced.
 		hasOutgoingFlights := hub.OutgoingFlights >= criteria.minOutgoingFlights
 
 		if hasOutgoingFlights {
@@ -573,8 +570,8 @@ func expandFromCityHub(g *Graph, from *Node, destCityCode string, date string, o
 
 		if foundEdges {
 			// Edges already added to graph during cache miss fetch
-			// Return cached data indicating success
-			return []byte("found_edges"), nil
+			// Return a marker indicating success; GetSearch caller only checks err != nil
+			return []byte("1"), nil
 		}
 
 		// Return error to trigger synthetic fallback
@@ -716,7 +713,10 @@ func (g *Graph) FindRoute(originID, destID string, opts SearchOptions) *Itinerar
 			// Expand from this node using lazy expansion
 			// Use the destination city code and date from search options for /search calls
 			if opts.DestCityCode != "" && opts.Date != "" {
-				g.ExpandGraphLazy(g.currentNodeByID(current.nodeID), opts.DestCityCode, opts.Date, &opts)
+				if err := g.ExpandGraphLazy(g.currentNodeByID(current.nodeID), opts.DestCityCode, opts.Date, &opts); err != nil {
+					// Expansion failed (e.g., API error, node not found) — fall back to synthetic edges
+					addSyntheticEdgesForNode(g, current.nodeID)
+				}
 			} else {
 				// If no dest city/code available, add synthetic edges as fallback
 				addSyntheticEdgesForNode(g, current.nodeID)