Add filter discovery endpoints with active/inactive status (Task 2.1)

- Add list_filters() and get_filter() to config_file_service.py:
  scans filter.d/, parses [Definition] + [Init] sections, merges .local
  overrides, and cross-references running jails to set active/used_by_jails
- Add FilterConfig.active, used_by_jails, source_file, has_local_override
  fields to the Pydantic model; add FilterListResponse and FilterNotFoundError
- Add GET /api/config/filters and GET /api/config/filters/{name} to config.py
- Remove the shadowed GET /api/config/filters list route from file_config.py;
  rename GET /api/config/filters/{name} raw variant to /filters/{name}/raw
- Update frontend: fetchFilterFiles() adapts FilterListResponse -> ConfFilesResponse;
  add fetchFilters() and fetchFilter() to api/config.ts; remove unused
  fetchFilterFiles/fetchActionFiles calls from useConfigActiveStatus
- Fix ConfigPageLogPath test mock to include fetchInactiveJails and related
  exports introduced by Stage 1
- Backend: 169 tests pass, mypy --strict clean, ruff clean
- Frontend: 63 tests pass, tsc --noEmit clean, eslint clean

Docs/Tasks.md

@@ -121,7 +121,7 @@ Currently BanGUI only shows jails that are actively running in fail2ban. fail2ba
 
 fail2ban ships with a large collection of filter definitions in `filter.d/` (over 80 files). Users need to see all available filters — both those currently in use by active jails and those available but unused — and assign them to jails.
 
-### Task 2.1 — Backend: List All Available Filters with Active/Inactive Status
+### Task 2.1 — Backend: List All Available Filters with Active/Inactive Status ✅ DONE
 
 **Goal:** Enumerate all filter config files and mark each as active or inactive.