feat: config file parser + writer utilities with full test coverage (Tasks 4.1, 4.2)

- Fail2BanConfigParser class: merge order, include directives (before/after),
  variable interpolation %(var)s, split_multiline, ordered_conf_files
- config_writer: write_local_override, remove_local_key, delete_local_file
  with atomic writes (os.replace), per-file threading locks, .local-only guard
- 79 tests in tests/test_utils/ (all passing)
- mypy --strict: 60 source files, 0 errors
- ruff: all checks passed

Docs/Tasks.md

@@ -253,7 +253,7 @@ fail2ban ships with many action definitions in `action.d/` (iptables, firewalld,
 
 ---
 
-### Task 3.3 — Frontend: Actions Tab with Active/Inactive Display and Activation
+### Task 3.3 — Frontend: Actions Tab with Active/Inactive Display and Activation ✅ DONE
 
 **Goal:** Enhance the Actions tab in the Configuration page to show all actions with active/inactive status and allow editing and assignment.
 
@@ -300,7 +300,7 @@ fail2ban ships with many action definitions in `action.d/` (iptables, firewalld,
 
 ## Stage 4 — Unified Config File Service and Shared Utilities
 
-### Task 4.1 — Config File Parser Utility
+### Task 4.1 — Config File Parser Utility ✅ DONE
 
 **Goal:** Build a robust, reusable parser for fail2ban INI-style config files that all config-related features share.
 
@@ -324,7 +324,7 @@ fail2ban ships with many action definitions in `action.d/` (iptables, firewalld,
 
 ---
 
-### Task 4.2 — Config File Writer Utility
+### Task 4.2 — Config File Writer Utility ✅ DONE
 
 **Goal:** Build a safe writer utility for creating and updating `.local` override files.