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

frontend/src/components/__tests__/ConfigPageLogPath.test.tsx

@@ -99,12 +99,23 @@ vi.mock("../../api/config", () => ({
   fetchFilterFile: vi.fn(),
   updateFilterFile: vi.fn(),
   createFilterFile: vi.fn(),
+  fetchFilters: vi.fn().mockResolvedValue({ filters: [], total: 0 }),
+  fetchFilter: vi.fn(),
   fetchActionFiles: mockFetchActionFiles,
   fetchActionFile: vi.fn(),
   updateActionFile: vi.fn(),
   createActionFile: vi.fn(),
   previewLog: vi.fn(),
   testRegex: vi.fn(),
+  fetchInactiveJails: vi.fn().mockResolvedValue({ jails: [], total: 0 }),
+  activateJail: vi.fn(),
+  deactivateJail: vi.fn(),
+  fetchParsedFilter: vi.fn(),
+  updateParsedFilter: vi.fn(),
+  fetchParsedAction: vi.fn(),
+  updateParsedAction: vi.fn(),
+  fetchParsedJailFile: vi.fn(),
+  updateParsedJailFile: vi.fn(),
 }));
 
 vi.mock("../../api/jails", () => ({

frontend/src/components/config/__tests__/FilterForm.test.tsx

@@ -23,6 +23,10 @@ const mockConfig: FilterConfig = {
   maxlines: null,
   datepattern: null,
   journalmatch: null,
+  active: false,
+  used_by_jails: [],
+  source_file: "/etc/fail2ban/filter.d/sshd.conf",
+  has_local_override: false,
 };
 
 function renderForm(name: string) {