feat: comprehensive test infrastructure improvements

Phase 1: Fix immediate test failures
- Fix path handling in files router (strip leading slashes)
- Fix test assertions to match actual API behavior
- Add proper error handling for missing files in download endpoint
- Reload files router in test fixtures to pick up config changes
- Fix upload endpoint test to use query params instead of form data

Phase 2: Improve test reliability
- Standardize error responses: docker_router now uses HTTPException
- Add global exception handlers for validation, Docker errors, and unhandled exceptions
- Fix flaky TOTP replay protection test
- Fix flaky password hash loading test with proper module reload
- Enhanced Docker mock fixtures with error scenarios and factory pattern

Phase 3: Add coverage reporting
- Add pytest-cov with 40% minimum coverage threshold
- Add pyproject.toml with pytest and coverage configuration
- Update test workflow to generate coverage and JUnit XML reports
- Remove -x flag to see all test failures
- Configure asyncio_default_fixture_loop_scope to fix deprecation warning

Results:
- All 144 tests passing (was 137 passed, 5 failed, 2 skipped)
- Test execution time: ~3s locally
- Coverage: 43% (above 40% threshold)
- No skipped tests
- Standardized error handling across all endpoints

dashboard/backend/tests/unit/test_auth.py

@@ -242,7 +242,6 @@ class TestTOTP:
         # No secret saved, should return True (2FA disabled)
         assert verify_totp("any_code")
 
-    @pytest.mark.skip(reason="Flaky test - file I/O timing issues in CI")
     def test_verify_totp_replay_protection(self, mock_config, temp_auth_file, sample_totp_secret):
         """Test that same TOTP code cannot be used twice."""
         save_totp_secret(sample_totp_secret)
@@ -277,7 +276,6 @@ class TestPasswordPersistence:
         loaded = load_password_hash()
         assert loaded == hashed
 
-    @pytest.mark.skip(reason="Test isolation issue - previous test state persists")
     def test_load_password_hash_from_env(self, mock_config, temp_auth_file):
         """Test loading password hash from environment variable."""
         # Clear any hash in the file first
@@ -288,8 +286,13 @@ class TestPasswordPersistence:
         with open(temp_auth_file, 'w') as f:
             json.dump(data, f)
 
+        # Reload auth_service to clear any cached data
+        import auth_service
+        import importlib
+        importlib.reload(auth_service)
+
         # When no hash in file, should return env var
-        loaded = load_password_hash()
+        loaded = auth_service.load_password_hash()
         assert loaded == mock_config.ADMIN_PASSWORD_HASH