feat(api): /deckies/deploy and /mutate become 202 fire-and-forget

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.

decnet/web/db/models/deploy.py

@@ -20,8 +20,12 @@ class DeployIniRequest(BaseModel):
 
 
 class DeployResponse(BaseModel):
+    """202-Accepted response: deploy spawned in background, client polls
+    GET /deckies/lifecycle?ids=... until each row reaches a terminal
+    status."""
     message: str
     mode: str
+    lifecycle_ids: list[str] = PydanticField(default_factory=list)
 
 
 class PurgeResponse(BaseModel):