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

backend/app/models/config.py

@@ -275,7 +275,15 @@ class MapColorThresholdsUpdate(BaseModel):
 
 
 class FilterConfig(BaseModel):
-    """Structured representation of a ``filter.d/*.conf`` file."""
+    """Structured representation of a ``filter.d/*.conf`` file.
+
+    The ``active``, ``used_by_jails``, ``source_file``, and
+    ``has_local_override`` fields are populated by
+    :func:`~app.services.config_file_service.list_filters` and
+    :func:`~app.services.config_file_service.get_filter`.  When the model is
+    returned from the raw file-based endpoints (``/filters/{name}/parsed``),
+    these fields carry their default values.
+    """
 
     model_config = ConfigDict(strict=True)
 
@@ -314,6 +322,33 @@ class FilterConfig(BaseModel):
         default=None,
         description="Systemd journal match expression.",
     )
+    # Active-status fields — populated by config_file_service.list_filters /
+    # get_filter; default to safe "inactive" values when not computed.
+    active: bool = Field(
+        default=False,
+        description=(
+            "``True`` when this filter is referenced by at least one currently "
+            "enabled (running) jail."
+        ),
+    )
+    used_by_jails: list[str] = Field(
+        default_factory=list,
+        description=(
+            "Names of currently enabled jails that reference this filter.  "
+            "Empty when ``active`` is ``False``."
+        ),
+    )
+    source_file: str = Field(
+        default="",
+        description="Absolute path to the ``.conf`` source file for this filter.",
+    )
+    has_local_override: bool = Field(
+        default=False,
+        description=(
+            "``True`` when a ``.local`` override file exists alongside the "
+            "base ``.conf`` file."
+        ),
+    )
 
 
 class FilterConfigUpdate(BaseModel):
@@ -335,6 +370,21 @@ class FilterConfigUpdate(BaseModel):
     journalmatch: str | None = Field(default=None)
 
 
+class FilterListResponse(BaseModel):
+    """Response for ``GET /api/config/filters``."""
+
+    model_config = ConfigDict(strict=True)
+
+    filters: list[FilterConfig] = Field(
+        default_factory=list,
+        description=(
+            "All discovered filters, each annotated with active/inactive status "
+            "and the jails that reference them."
+        ),
+    )
+    total: int = Field(..., ge=0, description="Total number of filters found.")
+
+
 # ---------------------------------------------------------------------------
 # Parsed action file models
 # ---------------------------------------------------------------------------