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/agent_app.py

@@ -59,7 +59,7 @@ def _map_docker(exc: DockerError):
         raise HTTPException(status_code=code, detail=exc.detail or exc.error)
     raise exc  # falls through to the global 502 DockerError handler
 
-AGENT_VERSION = "0.15.0"
+AGENT_VERSION = "0.16.0"
 
 
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