Consolidate domain exceptions into app.exceptions

Move all shared domain exception classes to backend/app/exceptions.py and update services/routers to import the canonical exceptions. Update docs to reflect the shared exceptions source.

backend/app/routers/jail_config.py

@@ -15,7 +15,12 @@ from app.dependencies import (
 from app.exceptions import (
     ConfigOperationError,
     ConfigValidationError,
+    ConfigWriteError,
+    JailAlreadyActiveError,
+    JailAlreadyInactiveError,
+    JailNameError,
     JailNotFoundError,
+    JailNotFoundInConfigError,
 )
 from app.models.config import (
     ActivateJailRequest,
@@ -30,13 +35,6 @@ from app.models.config import (
     RollbackResponse,
 )
 from app.services import config_service, jail_config_service, jail_service
-from app.services.jail_config_service import (
-    ConfigWriteError,
-    JailAlreadyActiveError,
-    JailAlreadyInactiveError,
-    JailNameError,
-    JailNotFoundInConfigError,
-)
 from app.utils.fail2ban_client import Fail2BanConnectionError
 
 router: APIRouter = APIRouter()