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.

docs/testing/backend-testing-improvement-plan.md

@@ -1,6 +1,6 @@
 # Backend Testing Improvement Plan
 
-**Status:** Phase 2 Complete ✅  
+**Status:** Phase 3 Complete ✅  
 **Created:** 2026-02-11  
 **Updated:** 2026-02-11  
 **Priority:** P0 - Critical
@@ -340,6 +340,43 @@ assert len(ws_manager.payloads) >= 2  # Started + completed
 
 ---
 
+## Phase 3 Completion Summary (2026-02-11)
+
+### Completed Items
+
+1. **Centralized Test Fixtures** ✅
+   - Added `mock_downloader` fixture to `tests/conftest.py`
+     - Configurable mock with `should_fail` and `return_value` attributes
+     - Records all download calls for verification
+   - Added `mock_websocket_manager` fixture to `tests/conftest.py`
+     - Recording WebSocket manager that captures all broadcast payloads
+     - Includes helper method `get_payloads_by_type()` for filtering
+   - Added `reset_singletons` autouse fixture to `tests/conftest.py`
+     - Resets DownloadManager, ServiceRegistry, ModelScanner, and SettingsManager
+     - Ensures test isolation and prevents singleton pollution
+
+2. **Split Large Test Files** ✅
+   - Split `tests/services/test_download_manager.py` (1422 lines) into:
+     - `test_download_manager_basic.py` - Core functionality (12 tests)
+     - `test_download_manager_error.py` - Error handling and execution (15 tests)
+     - `test_download_manager_concurrent.py` - Advanced scenarios (6 tests)
+   - Split `tests/utils/test_cache_paths.py` (530 lines) into:
+     - `test_cache_paths_resolution.py` - Path resolution and CacheType tests (11 tests)
+     - `test_cache_paths_validation.py` - Legacy path validation and cleanup (9 tests)
+     - `test_cache_paths_migration.py` - Migration scenarios and auto-cleanup (9 tests)
+
+3. **Complex Test Refactoring** ✅
+   - Reviewed `test_example_images_download_manager_unit.py`
+   - Existing async event-based patterns are appropriate for testing concurrent behavior
+   - No refactoring needed - tests follow consistent patterns and are maintainable
+
+### Test Results
+- **Download Manager Tests:** 33/33 passing across 3 files
+- **Cache Paths Tests:** 29/29 passing across 3 files
+- **Total Tests Maintained:** All existing tests preserved and organized
+
+---
+
 ## Phase 3: Architecture & Maintainability (P2) - Week 5-6
 
 ### 3.1 Centralize Test Fixtures
@@ -525,11 +562,11 @@ def test_cache_lookup_performance(benchmark):
 - [x] Strengthen assertions across integration tests (comprehensive assertions added)
 
 ### Week 5-6: Architecture
-- [ ] Add centralized fixtures to conftest.py
-- [ ] Split `test_download_manager.py` into 3 files
-- [ ] Split `test_cache_paths.py` into 3 files
-- [ ] Refactor complex test setups
-- [ ] Remove duplicate singleton reset fixtures
+- [x] Add centralized fixtures to conftest.py
+- [x] Split `test_download_manager.py` into 3 files
+- [x] Split `test_cache_paths.py` into 3 files
+- [x] Refactor complex test setups (reviewed - no changes needed)
+- [x] Remove duplicate singleton reset fixtures (consolidated in conftest.py)
 
 ### Week 7-8: Advanced Testing
 - [ ] Install hypothesis