skaldsong: enable PYTHONASYNCIODEBUG to surface orphaned-task warnings

Temporary diagnostic for the class of bug story 83ff386d47c6 hit
2026-05-23: POST /generation/start returned 202, then total silence
— no log, no DB state update, py-spy showed event loop idle with no
GenerationRunner frame anywhere. Strongly suggests a created_task()
result not held → GC'd → silent destroy.

PYTHONASYNCIODEBUG=1 emits "Task was destroyed but it is pending"
and "Task exception was never retrieved" warnings to stderr; that
should distinguish lost-task from cancelled-task on the next attempt.

Per skaldsong-dev's note, remove once they wire proper task-exception
capture upstream.

Diagnosis thread: althing 01KSBGKQBXA756JWW1KD4MPGXM

stacks/skaldsong/compose.yaml

@@ -78,6 +78,15 @@ services:
       # flat into /app/spa, not into /app/spa/dist).
       SKALDSONG_HOST_STATIC_ASSETS_PATH: /app/spa
 
+      # Temporary debug mode — surfaces "Task was destroyed but it is
+      # pending" + "Task exception was never retrieved" warnings to
+      # stderr. Added 2026-05-23 to diagnose a class of orphaned-task
+      # bugs in the GenerationRunner (story 83ff386d47c6 hit the
+      # symptom: 202 Accepted, then silent — no log, no DB update,
+      # event loop idle). Remove once skaldsong has proper
+      # task-exception capture wired (per skaldsong-dev's note).
+      PYTHONASYNCIODEBUG: "1"
+
       # Persistent state paths inside the container. The db/ parent
       # dir is the bind-mount target (not the file itself) so SQLite
       # can write its WAL + SHM siblings. Names MUST match what the