test: complete Phase 3 of backend testing improvement plan

Centralize test fixtures:
- Add mock_downloader fixture for configurable downloader mocking
- Add mock_websocket_manager fixture for WebSocket broadcast recording
- Add reset_singletons autouse fixture for test isolation
- Consolidate singleton cleanup in conftest.py

Split large test files:
- test_download_manager.py (1422 lines) → 3 focused files:
  - test_download_manager_basic.py: 12 core functionality tests
  - test_download_manager_error.py: 15 error handling tests
  - test_download_manager_concurrent.py: 6 advanced scenario tests

- test_cache_paths.py (530 lines) → 3 focused files:
  - test_cache_paths_resolution.py: 11 path resolution tests
  - test_cache_paths_validation.py: 9 legacy validation tests
  - test_cache_paths_migration.py: 9 migration scenario tests

Update documentation:
- Mark all Phase 3 checklist items as complete
- Add Phase 3 completion summary with test results

All 894 tests passing.

tests/utils/test_cache_paths_resolution.py

@@ -0,0 +1,149 @@
+"""Cache path resolution tests."""
+
+import os
+from pathlib import Path
+
+import pytest
+
+from py.utils.cache_paths import (
+    CacheType,
+    get_cache_base_dir,
+    get_cache_file_path,
+)
+
+
+class TestCacheType:
+    """Tests for the CacheType enum."""
+
+    def test_enum_values(self):
+        """Test that CacheType enum has correct values."""
+        assert CacheType.MODEL.value == "model"
+        assert CacheType.RECIPE.value == "recipe"
+        assert CacheType.RECIPE_FTS.value == "recipe_fts"
+        assert CacheType.TAG_FTS.value == "tag_fts"
+        assert CacheType.SYMLINK.value == "symlink"
+
+
+class TestGetCacheBaseDir:
+    """Tests for get_cache_base_dir function."""
+
+    def test_returns_cache_subdirectory(self):
+        """Test that cache base dir ends with 'cache'."""
+        cache_dir = get_cache_base_dir(create=True)
+        assert cache_dir.endswith("cache")
+        assert os.path.isdir(cache_dir)
+
+    def test_creates_directory_when_requested(self, tmp_path, monkeypatch):
+        """Test that directory is created when requested."""
+        settings_dir = tmp_path / "settings"
+        settings_dir.mkdir()
+
+        def fake_get_settings_dir(create: bool = True) -> str:
+            return str(settings_dir)
+
+        monkeypatch.setattr("py.utils.cache_paths.get_settings_dir", fake_get_settings_dir)
+
+        cache_dir = get_cache_base_dir(create=True)
+        assert os.path.isdir(cache_dir)
+        assert cache_dir == str(settings_dir / "cache")
+
+
+class TestGetCacheFilePath:
+    """Tests for get_cache_file_path function."""
+
+    def test_model_cache_path(self, tmp_path, monkeypatch):
+        """Test model cache file path generation."""
+        settings_dir = tmp_path / "settings"
+        settings_dir.mkdir()
+
+        def fake_get_settings_dir(create: bool = True) -> str:
+            return str(settings_dir)
+
+        monkeypatch.setattr("py.utils.cache_paths.get_settings_dir", fake_get_settings_dir)
+
+        path = get_cache_file_path(CacheType.MODEL, "my_library", create_dir=True)
+        expected = settings_dir / "cache" / "model" / "my_library.sqlite"
+        assert path == str(expected)
+        assert os.path.isdir(expected.parent)
+
+    def test_recipe_cache_path(self, tmp_path, monkeypatch):
+        """Test recipe cache file path generation."""
+        settings_dir = tmp_path / "settings"
+        settings_dir.mkdir()
+
+        def fake_get_settings_dir(create: bool = True) -> str:
+            return str(settings_dir)
+
+        monkeypatch.setattr("py.utils.cache_paths.get_settings_dir", fake_get_settings_dir)
+
+        path = get_cache_file_path(CacheType.RECIPE, "default", create_dir=True)
+        expected = settings_dir / "cache" / "recipe" / "default.sqlite"
+        assert path == str(expected)
+
+    def test_recipe_fts_path(self, tmp_path, monkeypatch):
+        """Test recipe FTS cache file path generation."""
+        settings_dir = tmp_path / "settings"
+        settings_dir.mkdir()
+
+        def fake_get_settings_dir(create: bool = True) -> str:
+            return str(settings_dir)
+
+        monkeypatch.setattr("py.utils.cache_paths.get_settings_dir", fake_get_settings_dir)
+
+        path = get_cache_file_path(CacheType.RECIPE_FTS, create_dir=True)
+        expected = settings_dir / "cache" / "fts" / "recipe_fts.sqlite"
+        assert path == str(expected)
+
+    def test_tag_fts_path(self, tmp_path, monkeypatch):
+        """Test tag FTS cache file path generation."""
+        settings_dir = tmp_path / "settings"
+        settings_dir.mkdir()
+
+        def fake_get_settings_dir(create: bool = True) -> str:
+            return str(settings_dir)
+
+        monkeypatch.setattr("py.utils.cache_paths.get_settings_dir", fake_get_settings_dir)
+
+        path = get_cache_file_path(CacheType.TAG_FTS, create_dir=True)
+        expected = settings_dir / "cache" / "fts" / "tag_fts.sqlite"
+        assert path == str(expected)
+
+    def test_symlink_path(self, tmp_path, monkeypatch):
+        """Test symlink cache file path generation."""
+        settings_dir = tmp_path / "settings"
+        settings_dir.mkdir()
+
+        def fake_get_settings_dir(create: bool = True) -> str:
+            return str(settings_dir)
+
+        monkeypatch.setattr("py.utils.cache_paths.get_settings_dir", fake_get_settings_dir)
+
+        path = get_cache_file_path(CacheType.SYMLINK, create_dir=True)
+        expected = settings_dir / "cache" / "symlink" / "symlink_map.json"
+        assert path == str(expected)
+
+    def test_sanitizes_library_name(self, tmp_path, monkeypatch):
+        """Test that library names are sanitized in paths."""
+        settings_dir = tmp_path / "settings"
+        settings_dir.mkdir()
+
+        def fake_get_settings_dir(create: bool = True) -> str:
+            return str(settings_dir)
+
+        monkeypatch.setattr("py.utils.cache_paths.get_settings_dir", fake_get_settings_dir)
+
+        path = get_cache_file_path(CacheType.MODEL, "my/bad:name", create_dir=True)
+        assert "my_bad_name" in path
+
+    def test_none_library_name_defaults_to_default(self, tmp_path, monkeypatch):
+        """Test that None library name defaults to 'default'."""
+        settings_dir = tmp_path / "settings"
+        settings_dir.mkdir()
+
+        def fake_get_settings_dir(create: bool = True) -> str:
+            return str(settings_dir)
+
+        monkeypatch.setattr("py.utils.cache_paths.get_settings_dir", fake_get_settings_dir)
+
+        path = get_cache_file_path(CacheType.MODEL, None, create_dir=True)
+        assert "default.sqlite" in path