Document route-to-region assignment and applied migration

Notes the new PUT /api/routes/{id}/region endpoint as a stand-in for
the still-blocked assignAreaToRegion, and removes the stale "migration
pending" caveat now that AddRegionManagement has been applied.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

sharedDocs/BACKEND_API_REFERENCE.md

@@ -584,10 +584,6 @@ All four of these endpoints return `404` if the region doesn't exist, the caller
 required region scope, or (for agency mapping) the target user doesn't hold the `RegionalAgency`
 role — the response body carries no distinction between "not found" and "forbidden".
 
-No EF Core migration for `RegionalTarget`/`RegionalBudget`/`RegionAgencyMap`/`RegionalTaxRule` has
-been applied yet — these endpoints will 500 against a database that hasn't run the pending
-migration.
-
 ---
 
 ### 6.6 Routes
@@ -600,6 +596,7 @@ migration.
 | `GET` | `/api/routes/{id}` | `routes.view` | Get route by ID |
 | `POST` | `/api/routes` | `routes.create` | Create route |
 | `PUT` | `/api/routes/{id}` | `routes.update` | Update route |
+| `PUT` | `/api/routes/{id}/region` | `routes.update` | Assign/reassign the route to a region |
 | `DELETE` | `/api/routes/{id}` | `routes.delete` | Delete route |
 
 **Create/Update request:**
@@ -627,6 +624,14 @@ migration.
 }
 ```
 
+**Assign to region request:**
+```json
+// PUT /api/routes/{id}/region
+{ "regionId": "guid" }
+```
+Returns the same `RouteResponse` shape as above. `404` if the route or the target region doesn't
+exist (or either is inactive).
+
 ---
 
 ### 6.7 Vehicles