Deploy console: real progress bar for image pulls (0.39.0)

Compose is now run with `--progress json` (probed once, falls back to the
plain text stream on older compose/agents). The console folds the event
stream into a weighted progress bar — download bytes per layer, then
container create/start — with a per-image bar and a byte/layer counter,
and renders the raw output one line per layer (updated in place) instead
of a wall of scrolling text.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/services/compose_service.py

@@ -361,13 +361,43 @@ async def stream_compose(
     await proc.wait()
 
 
+_json_progress: Optional[bool] = None
+
+
+async def supports_json_progress() -> bool:
+    """Whether this Docker Compose understands ``--progress json``.
+
+    The JSON progress stream carries per-layer ``current``/``total`` bytes, which
+    the deploy console turns into a real progress bar. Older compose releases
+    reject the value, so probe once (cheap, no side effects) and cache it; on a
+    negative result callers fall back to the plain text stream.
+    """
+    global _json_progress
+    if _json_progress is None:
+        try:
+            proc = await asyncio.create_subprocess_exec(
+                "docker", "compose", "--progress", "json", "version",
+                stdout=asyncio.subprocess.DEVNULL,
+                stderr=asyncio.subprocess.DEVNULL,
+            )
+            rc = await asyncio.wait_for(proc.wait(), timeout=15.0)
+            _json_progress = rc == 0
+        except Exception:  # noqa: BLE001 - probe failure just disables the feature
+            _json_progress = False
+    return _json_progress
+
+
 async def stream_up(stack_id: str, override: Optional[str] = None):
     """Run `compose up -d` streaming combined output, so the deploy console can
     show image-pull and container-create progress live.
 
     Yields ``("log", line)`` for each output line, then ``("done", returncode)``.
     """
-    cmd = _compose_base_cmd(stack_id, override) + ["up", "-d", "--remove-orphans"]
+    cmd = _compose_base_cmd(stack_id, override)
+    if await supports_json_progress():
+        # Global flag, must precede the subcommand.
+        cmd += ["--progress", "json"]
+    cmd += ["up", "-d", "--remove-orphans"]
     proc = await asyncio.create_subprocess_exec(
         *cmd,
         stdout=asyncio.subprocess.PIPE,