Phase 15: dashboard stack resource usage (0.16.0)

The dashboard now lists stacks in a table with live CPU and memory usage per
stack. Usage is sampled from docker stats (one-shot read per running container,
using the daemon-provided precpu for the CPU delta) and aggregated by compose
project.

- services/stats_service.py + GET /api/stacks/stats: per-stack cpu_used (cores),
  mem_used (bytes minus reclaimable cache), and the summed assigned cpu/mem
  limits (null when none set), read concurrently across containers.
- Dashboard: stacks render as a table with a CPU and a Memory meter. When a
  limit is assigned the bar fills toward it (used / limit + %); otherwise it
  fills toward the host total. Inline start/stop/restart per row for admins.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/routers/stacks.py

@@ -26,7 +26,7 @@ from models.setting import (
     EVENT_STACK_STOP,
 )
 from models.user import User
-from services import audit_service, compose_service, notify_service
+from services import audit_service, compose_service, notify_service, stats_service
 from services.convert_service import convert_docker_run
 
 router = APIRouter(prefix="/api/stacks", tags=["stacks"])
@@ -116,6 +116,12 @@ def create_stack(
     return _stack_summary(stack)
 
 
+@router.get("/stats")
+def stacks_stats(_user: User = Depends(get_current_user)) -> dict:
+    """Live CPU (cores) and memory usage per stack, with assigned limits."""
+    return stats_service.stack_stats()
+
+
 @router.get("/{stack_id}")
 def get_stack(
     stack_id: str,