This is the unblock for the wizard hang. Both endpoints used to run
docker compose synchronously inside the HTTP handler -- on master
(unihost) or via asyncio.gather of worker /deploy POSTs at 600s
timeout each (swarm) -- blocking every other API request.
New flow:
1. Commit the new config shape to repo state (fast).
2. Create one DeckyLifecycle row per decky (status=pending).
3. Spawn asyncio.create_task(run_deploy / run_mutate) -- the
lifecycle runner drives rows through running -> succeeded|failed
and emits decky.<name>.lifecycle on the bus.
4. Return 202 with {lifecycle_ids: [...]}. Wizard polls
GET /deckies/lifecycle?ids=... (next commit).
mutator/engine.py gains pick_new_services() -- shared between the
async API path and the watch-loop's synchronous mutate_decky().
DeployResponse grows lifecycle_ids[]. The old dispatch_decnet_config
helper still exists for the CLI swarm-deploy command path; it just
isn't called from the API handler anymore.
Test changes: 200 -> 202, drop dispatch_decnet_config mocks (handler
no longer calls it), assert lifecycle_ids in response + committed
state matches expectations.