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>

Docs/Architekture.md

@@ -135,11 +135,16 @@ backend/
 │   │   ├── geo_cache_flush.py #   Periodic geo cache persistence (dirty-set flush to SQLite)│  │   ├── geo_re_resolve.py  #   Periodic re-resolution of stale geo cache records│   │   └── health_check.py   #   Periodic fail2ban connectivity probe
 │   └── utils/                 # Helpers, constants, shared types
 │       ├── fail2ban_client.py #   Async wrapper around the fail2ban socket protocol
+│       ├── fail2ban_response.py #   Canonical response parsing: ok(), to_dict(), ensure_list(), is_not_found_error()
+│       ├── fail2ban_db_utils.py #   fail2ban database query helpers
 │       ├── ip_utils.py        #   IP/CIDR validation and normalisation
-│       ├── time_utils.py      #   Timezone-aware datetime helpers│       ├── jail_config.py     #   Jail config parser/serializer helper
-│       ├── conffile_parser.py #   Fail2ban config file parser/serializer
+│       ├── time_utils.py      #   Timezone-aware datetime helpers
+│       ├── config_file_utils.py #   fail2ban config file I/O
+│       ├── conffile_parser.py #   fail2ban config file parser/serializer
 │       ├── config_parser.py   #   Structured config object parser
-│       ├── config_writer.py   #   Atomic config file write operations│       └── constants.py       #   Shared constants (default paths, limits, etc.)
+│       ├── config_writer.py   #   Atomic config file write operations
+│       ├── jail_config.py     #   Jail config helper
+│       └── constants.py       #   Shared constants (default paths, limits, etc.)
 ├── tests/
 │   ├── conftest.py            # Shared fixtures (test app, client, mock DB)
 │   ├── test_routers/          # One test file per router