fix(engine): teardown(decky_id=...) built malformed service names

The nested list-comp `[f"{id}-{svc}" for svc in [d.services for d ...]]`
iterated over a list of lists, so `svc` was the whole services list and
f-string stringified it -> `decky3-['sip']`. docker compose saw "no such
service" and the per-decky teardown failed 500.

Flatten: find the matching decky once, then iterate its services. Noop
early on unknown decky_id and on empty service lists. Regression test
asserts the emitted compose args have no '[' or quote characters.

decnet/engine/deployer.py

@@ -195,10 +195,14 @@ def teardown(decky_id: str | None = None) -> None:
     client = docker.from_env()
 
     if decky_id:
-        svc_names = [f"{decky_id}-{svc}" for svc in [d.services for d in config.deckies if d.name == decky_id]]
-        if not svc_names:
+        decky = next((d for d in config.deckies if d.name == decky_id), None)
+        if decky is None:
             console.print(f"[red]Decky '{decky_id}' not found in current deployment.[/]")
             return
+        svc_names = [f"{decky_id}-{svc}" for svc in decky.services]
+        if not svc_names:
+            log.warning("teardown: decky %s has no services to stop", decky_id)
+            return
         _compose("stop", *svc_names, compose_file=compose_path)
         _compose("rm", "-f", *svc_names, compose_file=compose_path)
     else: