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/mutator/engine.py

@@ -37,6 +37,37 @@ log = get_logger("mutator")
 console = Console()
 
 
+def pick_new_services(decky: DeckyConfig) -> list[str] | None:
+    """Pick a fresh service list for *decky* using its archetype pool
+    (or the global pool when no archetype is set).  Returns ``None`` if
+    no services are available to pick from.
+
+    Pure: does not touch the repo, file system, or docker.  Shared by
+    the mutator watch loop and the async API handler.
+    """
+    if decky.archetype:
+        try:
+            arch = get_archetype(decky.archetype)
+            svc_pool = list(arch.services)
+        except ValueError:
+            svc_pool = all_service_names()
+    else:
+        svc_pool = all_service_names()
+
+    if not svc_pool:
+        return None
+
+    current_services = set(decky.services)
+    attempts = 0
+    while True:
+        count = random.randint(1, min(3, len(svc_pool)))  # nosec B311
+        chosen = set(random.sample(svc_pool, count))  # nosec B311
+        attempts += 1
+        if chosen != current_services or attempts > 20:
+            break
+    return list(chosen)
+
+
 @_traced("mutator.mutate_decky")
 async def mutate_decky(
     decky_name: str,