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/routers/docker_router.py

@@ -1,5 +1,5 @@
 import docker
-from fastapi import APIRouter, Depends, Query
+from fastapi import APIRouter, Depends, Query, HTTPException
 from config import DOCKER_HOST
 from rbac import require_admin
 
@@ -34,7 +34,7 @@ def list_containers():
 @router.post("/containers/{container_id}/{action}", dependencies=[Depends(require_admin())])
 def container_action(container_id: str, action: str):
     if action not in ("start", "stop", "restart"):
-        return {"error": "invalid action"}
+        raise HTTPException(status_code=400, detail="Invalid action. Must be one of: start, stop, restart")
     client = get_docker_client()
     c = client.containers.get(container_id)
     getattr(c, action)()