refactor: Extract fail2ban response utilities into shared module

Consolidate duplicate _ok(), _to_dict(), ensure_list(), and is_not_found_error()
functions from 6 service modules into a single canonical implementation at
backend/app/utils/fail2ban_response.py.

Changes:
- Create fail2ban_response.py with canonical implementations
- Remove local duplicates from: ban_service, jail_service, config_service,
  health_service, server_service, config_file_utils
- Update all imports to use shared module
- Add comprehensive docstrings and examples
- Update Architecture.md and Backend-Development.md documentation

Benefits:
- Single source of truth for response parsing logic
- Eliminates code duplication across service layer
- Improves maintainability and consistency
- Enables centralized bug fixes and improvements

Tests: All 228 service tests passing, no regressions

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

backend/app/services/server_service.py

@@ -17,6 +17,7 @@ import structlog
 from app.exceptions import ServerOperationError
 from app.models.server import ServerSettings, ServerSettingsResponse, ServerSettingsUpdate
 from app.utils.fail2ban_client import Fail2BanClient, Fail2BanCommand, Fail2BanResponse
+from app.utils.fail2ban_response import ok
 
 # ---------------------------------------------------------------------------
 # Types
@@ -60,27 +61,6 @@ def _to_str(value: object | None, default: str) -> str:
 # ---------------------------------------------------------------------------
 
 
-def _ok(response: Fail2BanResponse) -> object:
-    """Extract payload from a fail2ban ``(code, data)`` response.
-
-    Args:
-        response: Raw value returned by :meth:`~Fail2BanClient.send`.
-
-    Returns:
-        The payload ``data`` portion of the response.
-
-    Raises:
-        ValueError: If the return code indicates an error.
-    """
-    try:
-        code, data = response
-    except (TypeError, ValueError) as exc:
-        raise ValueError(f"Unexpected response shape: {response!r}") from exc
-    if code != 0:
-        raise ValueError(f"fail2ban error {code}: {data!r}")
-    return data
-
-
 async def _safe_get(
     client: Fail2BanClient,
     command: Fail2BanCommand,
@@ -98,7 +78,7 @@ async def _safe_get(
     """
     try:
         response = await client.send(command)
-        return _ok(cast("Fail2BanResponse", response))
+        return ok(cast("Fail2BanResponse", response))
     except Exception:
         return default
 
@@ -185,7 +165,7 @@ async def update_settings(socket_path: str, update: ServerSettingsUpdate) -> Non
     async def _set(key: str, value: Fail2BanSettingValue) -> None:
         try:
             response = await client.send(["set", key, value])
-            _ok(cast("Fail2BanResponse", response))
+            ok(cast("Fail2BanResponse", response))
         except ValueError as exc:
             raise ServerOperationError(f"Failed to set {key!r} = {value!r}: {exc}") from exc
 
@@ -220,7 +200,7 @@ async def flush_logs(socket_path: str) -> str:
     client = Fail2BanClient(socket_path=socket_path, timeout=_SOCKET_TIMEOUT)
     try:
         response = await client.send(["flushlogs"])
-        result = _ok(cast("Fail2BanResponse", response))
+        result = ok(cast("Fail2BanResponse", response))
         log.info("logs_flushed", result=result)
         return str(result)
     except ValueError as exc: