refactor(backend): clean up jail service, add error handling service

- Extract jail status/processing to helper functions
- Add error_handling.py service for centralized error handling
- Update config.py with validation and defaults
- Update .env.example with all config options
- Remove obsolete Tasks.md, add Service-Development.md
- Minor fixes across routers and services

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Docs/Backend-Development.md

@@ -1672,18 +1672,50 @@ async def get_jail(...) -> JailDetailResponse:
 - Map domain exceptions to HTTP status codes via FastAPI **exception handlers** registered on the app.
 - Always log errors with context before raising.
 
-```python
-class JailNotFoundError(Exception):
-    def __init__(self, name: str) -> None:
-        self.name: str = name
-        super().__init__(f"Jail '{name}' not found")
+### Service Error Contracts
 
-# In main.py
-@app.exception_handler(JailNotFoundError)
-async def jail_not_found_handler(request: Request, exc: JailNotFoundError) -> JSONResponse:
-    return JSONResponse(status_code=404, content={"detail": f"Jail '{exc.name}' not found"})
+Each service method must document which error handling pattern it follows. This
+lets callers know what to expect without reading the implementation. See
+`Docs/Service-Development.md` for the full guide.
+
+**ABORT_ON_ERROR** — Raise an exception, let the router handle it. Used for:
+auth, writes, state changes, any operation where partial success is meaningless.
+
+**RETURN_DEFAULT** — Return empty result and log warning. Never raises. Used for:
+informational reads where infrastructure unavailability should not block the UI.
+
+**PARTIAL_RESULT** — Return a result that contains both successful items and a
+list of errors. Caller decides what to do with each.
+
+```python
+async def get_settings(socket_path: str) -> DomainServerSettingsResult:
+    """Return current fail2ban server-level settings.
+
+    Error contract: RETURN_DEFAULT. Returns DomainServerSettingsResult with
+    default values if socket is unreachable. Never raises.
+    """
+    ...
+
+async def start_jail(socket_path: str, name: str) -> None:
+    """Start a stopped fail2ban jail.
+
+    Error contract: ABORT_ON_ERROR. Raises JailNotFoundError (404),
+    JailOperationError (409), Fail2BanConnectionError (503).
+    """
+    ...
 ```
 
+```python
+class ServiceErrorContract:
+    """
+    ABORT_ON_ERROR: Raise exception, let router handle
+    RETURN_DEFAULT: Return empty result, log warning
+    PARTIAL_RESULT: Return partial success with error list
+    """
+```
+
+The error contract enum and helper are in `app.services.error_handling`.
+
 ### Routers and Exception Propagation
 
 - **Routers must NOT construct `HTTPException` for domain errors** — let domain exceptions propagate.