fix(infra): bump Postgres shm_size (vacuum DiskFull) + raise DB-backup time limit

Two more maintenance-queue failures from the operator's 24h list:
- vacuum_analyze died with "could not resize shared memory segment to 67MB: No
  space left on device" — Docker's default /dev/shm is 64MB, too small for
  VACUUM (ANALYZE)'s parallel-worker shared memory. Set the postgres service
  shm_size: 512m.
- backup_db_task timed out at its 12-min limit once the DB grew; a pg_dump can't
  be chunked, so raise it to 30/35 min. (A long backup still briefly holds the
  concurrency-1 lane — the structural fix is a dedicated lane for long one-shots.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/tasks/backup.py

@@ -41,7 +41,11 @@ def _mark_failed(session, row: BackupRun, exc: BaseException) -> None:
     bind=True,
     autoretry_for=(OperationalError, DBAPIError),
     retry_backoff=10, retry_backoff_max=120, max_retries=2,
-    soft_time_limit=600, time_limit=720,
+    # A pg_dump can't be chunked; the 12-min limit timed out once the DB grew
+    # (operator-flagged 2026-06-07). 30/35 min gives real headroom. (A long
+    # backup still briefly holds the concurrency-1 maintenance lane — the
+    # structural fix is a dedicated lane for the long one-shots.)
+    soft_time_limit=1800, time_limit=2100,
 )
 def backup_db_task(self, *, tag: str | None = None,
                    triggered_by: str = "manual") -> dict:

docker-compose.yml

@@ -21,6 +21,11 @@ services:
 
   postgres:
     image: pgvector/pgvector:pg16
+    # Docker's default /dev/shm is 64MB; VACUUM (ANALYZE) and parallel queries
+    # allocate larger shared-memory segments and fail with
+    # "could not resize shared memory segment ... No space left on device"
+    # (operator-flagged 2026-06-07, vacuum_analyze on import_task needed 67MB).
+    shm_size: 512m
     environment:
       POSTGRES_USER: ${DB_USER:-fabledcurator}
       POSTGRES_PASSWORD: ${DB_PASSWORD:-fabledcurator_dev}