Add assignment engine, FCM, WebSockets, city gate, and internal APIs

- internal/assignment: GEORADIUS miler assignment with retry/escalation,
  customer-side provider scoring, FCM notifications on assign
- internal/notify: Firebase Admin SDK (FCM) client initialisation
- internal/ws: WebSocket handlers for live parcel tracking and
  customer↔miler chat
- middlewares: city gate (pincode prefix validation), internal API key
  auth, WebSocket JWT auth
- controllers: InternalNotify + InternalReassign for machine-to-machine
  calls; pricing helpers wired into CreateCustomerBooking and CreateCRMBooking
- routes: /internal/*, /ws/bookings/:id/track, /ws/bookings/:id/chat
- models/users, models/doormile_pricing: new fields for device tokens,
  assignment state, pricing bands
- seed_data.sql: initial pricing seed rows

.env and Firebase service-account JSON intentionally excluded.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/notify/fcm.go

@@ -0,0 +1,74 @@
+package notify
+
+import (
+	"context"
+	"os"
+	"sync"
+	"time"
+
+	firebase "firebase.google.com/go/v4"
+	"firebase.google.com/go/v4/messaging"
+	"doormile/utils"
+	"google.golang.org/api/option"
+)
+
+var (
+	fcmClient *messaging.Client
+	fcmOnce   sync.Once
+)
+
+// InitFCM initializes the Firebase Admin SDK using the service account JSON file
+// at FIREBASE_SERVICE_ACCOUNT_PATH. Safe to call from main() at startup.
+// If the env var is unset or the file is invalid, FCM is disabled for the process
+// lifetime — all SendToDevice calls become no-ops.
+func InitFCM() {
+	fcmOnce.Do(func() {
+		path := os.Getenv("FIREBASE_SERVICE_ACCOUNT_PATH")
+		if path == "" {
+			utils.Warn("FCM: FIREBASE_SERVICE_ACCOUNT_PATH not set — push notifications disabled")
+			return
+		}
+
+		ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+		defer cancel()
+
+		app, err := firebase.NewApp(ctx, nil, option.WithCredentialsFile(path))
+		if err != nil {
+			utils.Error("FCM: failed to initialize Firebase app", "error", err)
+			return
+		}
+
+		client, err := app.Messaging(ctx)
+		if err != nil {
+			utils.Error("FCM: failed to get Messaging client", "error", err)
+			return
+		}
+
+		fcmClient = client
+		utils.Info("FCM: initialized successfully")
+	})
+}
+
+// SendToDevice sends an FCM notification to a single device token.
+// Returns nil (no-op) if FCM was not initialized or the token is empty.
+// The caller should log errors but must not block the booking flow on failure.
+func SendToDevice(token, title, body string, data map[string]string) error {
+	if fcmClient == nil || token == "" {
+		return nil
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	msg := &messaging.Message{
+		Notification: &messaging.Notification{
+			Title: title,
+			Body:  body,
+		},
+		Data:  data,
+		Token: token,
+	}
+
+	_, err := fcmClient.Send(ctx, msg)
+	return err
+}