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/routes/test_recipe_query_handler.py

@@ -5,6 +5,7 @@ from types import SimpleNamespace
 import pytest
 
 from py.routes.handlers.recipe_handlers import RecipeQueryHandler
+from py.services.recipe_scanner import UNKNOWN_BASE_MODEL_FILTER
 
 
 async def _noop():
@@ -46,3 +47,42 @@ async def test_recipe_query_handler_base_models_limit_zero_returns_all():
         {"name": "SDXL", "count": 2},
         {"name": "LTXV 2.3", "count": 1},
     ]
+
+
+@pytest.mark.asyncio
+async def test_recipe_query_handler_base_models_includes_unknown_bucket():
+    cache = SimpleNamespace(
+        raw_data=[
+            {"base_model": "SDXL"},
+            {"base_model": None},
+            {"base_model": ""},
+        ]
+    )
+    scanner = SimpleNamespace(get_cached_data=lambda: None)
+
+    async def get_cached_data():
+        return cache
+
+    scanner.get_cached_data = get_cached_data
+
+    handler = RecipeQueryHandler(
+        ensure_dependencies_ready=_noop,
+        recipe_scanner_getter=lambda: scanner,
+        format_recipe_file_url=lambda value: value,
+        logger=logging.getLogger(__name__),
+    )
+
+    response = await handler.get_base_models(
+        SimpleNamespace(query={"limit": "0"})  # pyright: ignore[reportArgumentType]
+    )
+    text = response.text
+    assert text is not None
+    payload = json.loads(text)
+
+    assert payload["success"] is True
+    # Unknown bucket carries a dedicated marker so the UI can show "Unknown"
+    # without colliding with real base model strings.
+    assert payload["base_models"] == [
+        {"name": "Unknown", "value": UNKNOWN_BASE_MODEL_FILTER, "count": 2},
+        {"name": "SDXL", "count": 1},
+    ]