Add /api/v1/ip/<ip> endpoint for IP to AS lookups

- Add handleIPLookup handler that uses GetASInfoForIP
- Create writeJSONError and writeJSONSuccess helper functions
- Refactor all JSON error responses to use the helpers
- Add GetASInfoForIP to Store interface
- Add mock implementation for tests
- Fix all linter warnings

internal/routewatch/app_integration_test.go

@@ -187,6 +187,17 @@ func (m *mockStore) GetLiveRouteCounts() (ipv4Count, ipv6Count int, err error) {
 	return m.RouteCount / 2, m.RouteCount / 2, nil
 }
 
+// GetASInfoForIP mock implementation
+func (m *mockStore) GetASInfoForIP(ip string) (*database.ASInfo, error) {
+	// Simple mock - return a test AS
+	return &database.ASInfo{
+		ASN:         15169,
+		Handle:      "GOOGLE",
+		Description: "Google LLC",
+		Prefix:      "8.8.8.0/24",
+	}, nil
+}
+
 func TestRouteWatchLiveFeed(t *testing.T) {
 
 	// Create mock database