feat(workshop): vehicles must be assigned to a customer

- Added customer_id column to fleet_vehicles with FK to customers.

- Enforced customer_id on create_vehicle endpoints (workshop and fleet).

- New vehicle modal now requires a selected customer and stores customer_id.

- Migration v4.13 applied to all tenants.

pos/migrations/v4.13_fleet_vehicle_customer.sql

@@ -0,0 +1,20 @@
+-- v4.13_fleet_vehicle_customer.sql
+-- Link fleet vehicles to a customer.
+
+ALTER TABLE fleet_vehicles
+    ADD COLUMN IF NOT EXISTS customer_id INTEGER;
+
+DO $$
+BEGIN
+    IF NOT EXISTS (
+        SELECT 1 FROM pg_constraint
+        WHERE conname = 'fleet_vehicles_customer_id_fkey'
+    ) THEN
+        ALTER TABLE fleet_vehicles
+            ADD CONSTRAINT fleet_vehicles_customer_id_fkey
+            FOREIGN KEY (customer_id) REFERENCES customers(id);
+    END IF;
+END $$;
+
+CREATE INDEX IF NOT EXISTS idx_fleet_vehicles_customer_id
+    ON fleet_vehicles (customer_id);