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>

middlewares/internal_auth.go

@@ -0,0 +1,21 @@
+package middlewares
+
+import (
+	"os"
+
+	"github.com/gofiber/fiber/v2"
+)
+
+// InternalKeyAuth guards machine-to-machine endpoints with a static API key.
+// The key is read from INTERNAL_API_KEY env var at request time so it can be
+// rotated without redeployment. Returns 401 if the header is missing or wrong.
+func InternalKeyAuth(c *fiber.Ctx) error {
+	expected := os.Getenv("INTERNAL_API_KEY")
+	if expected == "" || c.Get("X-Internal-Key") != expected {
+		return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{
+			"success": false,
+			"message": "unauthorized",
+		})
+	}
+	return c.Next()
+}