revert: restore GatewayRead.token field to avoid frontend breaking change

The has_token boolean redaction requires coordinated frontend changes
(detail page, edit page, orval types). Revert to returning the raw
token for now; token redaction will be handled in a dedicated PR.

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

backend/app/schemas/gateways.py

@@ -65,7 +65,7 @@ class GatewayRead(GatewayBase):
 
     id: UUID
     organization_id: UUID
-    has_token: bool = False
+    token: str | None = None
     created_at: datetime
     updated_at: datetime