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/action_config.py

@@ -2,10 +2,18 @@ from __future__ import annotations
 
 from typing import Annotated
 
-from fastapi import APIRouter, HTTPException, Path, Query, status
+from fastapi import APIRouter, HTTPException, Path, Query, Request, status
 
 from app.dependencies import AuthDep, Fail2BanConfigDirDep, Fail2BanSocketDep
-from app.exceptions import ConfigWriteError
+from app.exceptions import (
+    ActionAlreadyExistsError,
+    ActionNameError,
+    ActionNotFoundError,
+    ActionReadonlyError,
+    ConfigWriteError,
+    JailNameError,
+    JailNotFoundInConfigError,
+)
 from app.models.config import (
     ActionConfig,
     ActionCreateRequest,
@@ -14,13 +22,6 @@ from app.models.config import (
     AssignActionRequest,
 )
 from app.services import action_config_service
-from app.services.action_config_service import (
-    ActionAlreadyExistsError,
-    ActionNameError,
-    ActionNotFoundError,
-    ActionReadonlyError,
-)
-from app.services.jail_config_service import JailNameError, JailNotFoundInConfigError
 
 router: APIRouter = APIRouter()
 
@@ -29,6 +30,11 @@ _ActionNamePath = Annotated[
     Path(description='Action base name, e.g. ``iptables`` or ``iptables.conf``.'),
 ]
 
+_NamePath = Annotated[
+    str,
+    Path(description='Jail name as configured in fail2ban.'),
+]
+
 
 def _action_not_found(name: str) -> HTTPException:
     return HTTPException(
@@ -36,6 +42,17 @@ def _action_not_found(name: str) -> HTTPException:
         detail=f"Action not found: {name!r}",
     )
 
+
+def _bad_request(message: str) -> HTTPException:
+    return HTTPException(status_code=status.HTTP_400_BAD_REQUEST, detail=message)
+
+
+def _not_found(name: str) -> HTTPException:
+    return HTTPException(
+        status_code=status.HTTP_404_NOT_FOUND,
+        detail=f"Jail not found: {name!r}",
+    )
+
 @router.get(
     "/actions",
     response_model=ActionListResponse,