Volumes page: on-demand volume sizes (0.18.0)

Docker's volume list has no size, so add a "Compute sizes" button that runs
`docker system df` (via client.df()) and shows per-volume size in a new Size
column. The df walk is expensive (seconds), so results are cached ~60s and
loaded on demand instead of on every poll.

- volume_service.volume_sizes(force) with a 60s TTL cache; GET /api/volumes/sizes
  + agent /agent/volumes/sizes + proxy /api/agents/{id}/volumes/sizes.
- Frontend: volumesApi.sizes(force, agentId); Volumes page gained a Size column
  and a Compute sizes button (per host) that triggers the lookup.

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

backend/services/volume_service.py

@@ -1,6 +1,8 @@
 """Docker volume management + Compose volume YAML generation."""
 from __future__ import annotations
 
+import threading
+import time
 from typing import Literal, Optional
 
 import yaml
@@ -9,6 +11,33 @@ from docker_client import get_client, safe_call
 
 COMPOSE_PROJECT_LABEL = "com.docker.compose.project"
 
+# `docker system df -v` walks every volume's contents, so it can take many
+# seconds. Cache the result so the (polled) UI and repeated requests reuse it.
+_SIZE_TTL = 60.0
+_size_cache: dict = {"at": 0.0, "data": {}}
+_size_lock = threading.Lock()
+
+
+def volume_sizes(force: bool = False) -> dict:
+    """Return {volume_name: size_bytes|None}. Cached (~60s) since it's expensive."""
+    now = time.time()
+    with _size_lock:
+        if not force and _size_cache["data"] and now - _size_cache["at"] < _SIZE_TTL:
+            return _size_cache["data"]
+
+    client = get_client()
+    df = safe_call(client.df)
+    sizes: dict[str, Optional[int]] = {}
+    for v in df.get("Volumes") or []:
+        ud = v.get("UsageData") or {}
+        size = ud.get("Size")
+        sizes[v.get("Name")] = size if isinstance(size, int) and size >= 0 else None
+
+    with _size_lock:
+        _size_cache["at"] = time.time()
+        _size_cache["data"] = sizes
+    return sizes
+
 
 # --------------------------------------------------------------------------- #
 # Listing / pruning