feat(metadata): implement fallback provider strategy for deleted models

Refactor metadata sync service to use a prioritized provider fallback system when handling deleted CivitAI models. The new approach:

1. Attempts civarchive_api provider first for deleted models
2. Falls back to sqlite provider if archive DB is enabled
3. Maintains existing default provider behavior for non-deleted models
4. Tracks provider attempts and errors for better debugging

This improves reliability when fetching metadata for deleted models by trying multiple sources before giving up, and provides clearer error messages based on which providers were attempted.

tests/services/test_metadata_sync_service.py

@@ -32,6 +32,8 @@ def build_service(
         get_model_by_hash=AsyncMock(),
         get_model_version=AsyncMock(),
     )
+    if default_provider is None:
+        provider.get_model_by_hash.return_value = (None, None)
 
     default_provider_factory = AsyncMock(return_value=provider)
     provider_selector = provider_selector or AsyncMock(return_value=provider)
@@ -138,6 +140,7 @@ async def test_fetch_and_update_model_success_updates_cache(tmp_path):
     assert model_data["from_civitai"] is True
     assert model_data["civitai_deleted"] is False
     assert "civitai" in model_data
+    assert model_data["metadata_source"] == "civitai_api"
 
     helpers.metadata_manager.hydrate_model_data.assert_not_awaited()
     assert model_data["hydrated"] is True
@@ -219,6 +222,124 @@ async def test_fetch_and_update_model_respects_deleted_without_archive():
     update_cache.assert_not_awaited()
 
 
+@pytest.mark.asyncio
+async def test_fetch_and_update_model_prefers_civarchive_for_deleted_models(tmp_path):
+    default_provider = SimpleNamespace(
+        get_model_by_hash=AsyncMock(),
+        get_model_version=AsyncMock(),
+    )
+    civarchive_provider = SimpleNamespace(
+        get_model_by_hash=AsyncMock(
+            return_value=(
+                {
+                    "source": "civarchive",
+                    "model": {"name": "Recovered", "description": "", "tags": []},
+                    "images": [],
+                    "baseModel": "sdxl",
+                },
+                None,
+            )
+        ),
+        get_model_version=AsyncMock(),
+    )
+
+    async def select_provider(name: str):
+        return civarchive_provider if name == "civarchive_api" else default_provider
+
+    provider_selector = AsyncMock(side_effect=select_provider)
+    helpers = build_service(
+        settings_values={"enable_metadata_archive_db": False},
+        default_provider=default_provider,
+        provider_selector=provider_selector,
+    )
+
+    model_path = tmp_path / "model.safetensors"
+    model_data = {
+        "civitai_deleted": True,
+        "metadata_source": "civarchive",
+        "civitai": {"source": "civarchive"},
+        "file_path": str(model_path),
+    }
+    update_cache = AsyncMock()
+
+    ok, error = await helpers.service.fetch_and_update_model(
+        sha256="deadbeef",
+        file_path=str(model_path),
+        model_data=model_data,
+        update_cache_func=update_cache,
+    )
+
+    assert ok
+    assert error is None
+    provider_selector.assert_awaited_with("civarchive_api")
+    helpers.default_provider_factory.assert_not_awaited()
+    civarchive_provider.get_model_by_hash.assert_awaited_once_with("deadbeef")
+    update_cache.assert_awaited()
+    assert model_data["metadata_source"] == "civarchive"
+    helpers.metadata_manager.save_metadata.assert_awaited()
+
+
+@pytest.mark.asyncio
+async def test_fetch_and_update_model_falls_back_to_sqlite_after_civarchive_failure(tmp_path):
+    default_provider = SimpleNamespace(
+        get_model_by_hash=AsyncMock(),
+        get_model_version=AsyncMock(),
+    )
+    civarchive_provider = SimpleNamespace(
+        get_model_by_hash=AsyncMock(return_value=(None, "Model not found")),
+        get_model_version=AsyncMock(),
+    )
+    sqlite_payload = {
+        "source": "archive_db",
+        "model": {"name": "Recovered", "description": "", "tags": []},
+        "images": [],
+        "baseModel": "sdxl",
+    }
+    sqlite_provider = SimpleNamespace(
+        get_model_by_hash=AsyncMock(return_value=(sqlite_payload, None)),
+        get_model_version=AsyncMock(),
+    )
+
+    async def select_provider(name: str):
+        if name == "civarchive_api":
+            return civarchive_provider
+        if name == "sqlite":
+            return sqlite_provider
+        return default_provider
+
+    provider_selector = AsyncMock(side_effect=select_provider)
+    helpers = build_service(
+        settings_values={"enable_metadata_archive_db": True},
+        default_provider=default_provider,
+        provider_selector=provider_selector,
+    )
+
+    model_path = tmp_path / "model.safetensors"
+    model_data = {
+        "civitai_deleted": True,
+        "db_checked": False,
+        "file_path": str(model_path),
+    }
+    update_cache = AsyncMock()
+
+    ok, error = await helpers.service.fetch_and_update_model(
+        sha256="cafe",
+        file_path=str(model_path),
+        model_data=model_data,
+        update_cache_func=update_cache,
+    )
+
+    assert ok and error is None
+    assert civarchive_provider.get_model_by_hash.await_count == 1
+    assert sqlite_provider.get_model_by_hash.await_count == 1
+    assert model_data["metadata_source"] == "archive_db"
+    assert model_data["db_checked"] is True
+    assert provider_selector.await_args_list[0].args == ("civarchive_api",)
+    assert provider_selector.await_args_list[1].args == ("sqlite",)
+    update_cache.assert_awaited()
+    helpers.metadata_manager.save_metadata.assert_awaited()
+
+
 @pytest.mark.asyncio
 async def test_relink_metadata_fetches_version_and_updates_sha(tmp_path):
     provider = SimpleNamespace(