Stream update progress into a bar on the stack's row (0.42.0)

Update ran as a blocking POST with nothing to show but a spinner, so the
status added in 86c67df could only sit above the table as a banner.

Adds /ws/update/{stack_id}, streaming `compose pull` then `up -d` with
--progress json, and feeds it through the existing DeployTracker — the
same weighting the deploy console uses. The result renders as a progress
bar inside the stack's own row: percentage, phase label, and layer/byte
detail. Non-streaming actions (start/stop/restart/pull/down) reuse the
bar in its indeterminate form, so every row action looks consistent.

compose_service gains _stream_phase, shared by stream_up and the new
stream_update; a failed pull short-circuits before `up`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016pMmFFkdfxkoYjcEcpZTa5

backend/version.py

@@ -1,3 +1,3 @@
 """Single source of truth for the StackPilot release version."""
 
-APP_VERSION = "0.41.0"
+APP_VERSION = "0.42.0"