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/log_service.py

@@ -8,7 +8,6 @@ from __future__ import annotations
 import asyncio
 import re
 from pathlib import Path
-from typing import cast
 
 import structlog
 
@@ -26,8 +25,8 @@ from app.utils.fail2ban_client import (
     Fail2BanClient,
     Fail2BanConnectionError,
     Fail2BanProtocolError,
-    Fail2BanResponse,
 )
+from app.utils.fail2ban_response import ok
 
 log: structlog.stdlib.BoundLogger = structlog.get_logger()
 
@@ -40,21 +39,6 @@ _NON_FILE_LOG_TARGETS: frozenset[str] = frozenset(
 _SAFE_LOG_PREFIXES: tuple[str, ...] = ("/var/log", "/config/log")
 
 
-def _ok(response: object) -> object:
-    """Extract the payload from a fail2ban ``(return_code, data)`` response."""
-    try:
-        code, data = cast("Fail2BanResponse", response)
-    except (TypeError, ValueError) as exc:
-        raise ValueError(
-            f"Unexpected fail2ban response shape: {response!r}"
-        ) from exc
-
-    if code != 0:
-        raise ValueError(f"fail2ban returned error code {code}: {data!r}")
-
-    return data
-
-
 def _count_file_lines(file_path: str) -> int:
     """Count the total number of lines in *file_path* synchronously."""
     count = 0
@@ -71,7 +55,7 @@ async def _safe_get(
 ) -> object | None:
     """Send a command and return *default* if it fails."""
     try:
-        return _ok(await client.send(command))
+        return ok(await client.send(command))
     except (
         Fail2BanConnectionError,
         Fail2BanProtocolError,