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/types/config.ts

@@ -274,6 +274,29 @@ export interface FilterConfig {
   datepattern: string | null;
   /** journalmatch, or null. */
   journalmatch: string | null;
+  /**
+   * True when this filter is referenced by at least one currently running jail.
+   * Defaults to false when the status was not computed (e.g. /parsed endpoint).
+   */
+  active: boolean;
+  /**
+   * Names of currently enabled jails that reference this filter.
+   * Empty when active is false.
+   */
+  used_by_jails: string[];
+  /** Absolute path to the .conf source file. Empty string when not computed. */
+  source_file: string;
+  /** True when a .local override file exists alongside the base .conf. */
+  has_local_override: boolean;
+}
+
+/**
+ * Response for GET /api/config/filters.
+ * Lists all discovered filters with active/inactive status.
+ */
+export interface FilterListResponse {
+  filters: FilterConfig[];
+  total: number;
 }
 
 /**