Let a stuck backup be stopped instead of stranding the stack (0.67.0)

A backup holds the stack lock for up to twelve hours — a large volume
legitimately takes that long — and stops the stack's containers for a
consistent snapshot. One that died without unwinding therefore left the
stack stopped and refusing every start, stop, restart and deploy with
"Stack 'npmplus' is busy: backup in progress", with nothing to click
until the TTL ran out.

Backups and restores now register themselves in stack_job_service while
they run, with their phase and the helper containers they stream
through. Cancelling sets a flag and removes those containers, which is
what actually ends the long read: the backup then unwinds through its
own finally — the half-written archive goes, the containers it stopped
are started again, the lock is released. Checks sit between two files
and two volumes, never inside one. A restore is not cancellable at all;
it is halfway through writing a stack's data back.

POST /api/stacks/{id}/unlock does that and waits for it. A backup lock
with no job behind it is a leftover, so it is released on one click; a
deploy lock takes an explicit force, because compose keeps running past
its request. GET /api/stacks/{id}/lock and the stack detail response say
what holds the stack and since when, and the stack page turns that into
"Stop backup" / "Release lock".

Startup now clears every lock rather than only expired ones: after a
restart nothing of ours is in flight, the same reasoning the leftover
helper containers already follow.

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

backend/routers/backups.py

@@ -65,8 +65,11 @@ def _summary(report: dict) -> str:
 
 
 def _backup_http_error(exc: backup_service.BackupError) -> HTTPException:
-    # 409 for a stack that is busy or already exists, 400 for the rest.
-    conflict = isinstance(exc, backup_service.StackBusyError) or "already exists" in str(exc)
+    # 409 for a stack that is busy, a backup somebody stopped, or a target that
+    # already exists; 400 for the rest.
+    conflict = isinstance(
+        exc, (backup_service.StackBusyError, backup_service.BackupCancelled)
+    ) or "already exists" in str(exc)
     return HTTPException(status_code=409 if conflict else 400, detail=str(exc))