feat(recipes): add Unknown base-model filter bucket for undetermined recipes

Normalize undetermined recipe base_model to None in RecipeFormatParser
(previously ''). get_base_models now reports an "Unknown" bucket backed
by a dedicated __unknown__ marker, and the listing filter matches it
against recipes whose base model is falsy. Frontend renders the bucket
label as "Unknown" while filtering via the marker.

Tests: handler, scanner, parser, and frontend filtering.

tests/services/test_recipe_format_parser.py

@@ -111,6 +111,36 @@ async def test_recipe_format_parser_populates_checkpoint(monkeypatch):
     assert result["model"] == checkpoint
 
 
+@pytest.mark.asyncio
+async def test_recipe_format_parser_base_model_defaults_to_none_when_unknown(monkeypatch):
+    class _FakeScanner:
+        pass
+
+    # No checkpoint and empty base_model -> unknown renders as None (not "")
+    result = await _parse(
+        monkeypatch,
+        {"title": "T", "base_model": "", "loras": [], "gen_params": {}},
+        _FakeScanner(),
+    )
+    assert result["base_model"] is None
+
+    # Missing base_model key behaves the same
+    result = await _parse(
+        monkeypatch,
+        {"title": "T", "loras": [], "gen_params": {}},
+        _FakeScanner(),
+    )
+    assert result["base_model"] is None
+
+    # A real base_model in recipe metadata is kept
+    result = await _parse(
+        monkeypatch,
+        {"title": "T", "base_model": "Illustrious", "loras": [], "gen_params": {}},
+        _FakeScanner(),
+    )
+    assert result["base_model"] == "Illustrious"
+
+
 @pytest.mark.asyncio
 async def test_recipe_format_parser_marks_lora_in_library_by_version(monkeypatch):
     async def fake_metadata_provider():