fix(infra): size Postgres /dev/shm via tmpfs mount (shm_size ignored under Swarm)

The prod stack runs under Docker Swarm (docker stack deploy), which SILENTLY
IGNORES `shm_size` — container inspect showed ShmSize still 64MB after the
a183be7 fix, and vacuum_analyze kept hitting DiskFull resizing a ~64MB POSIX
DSM segment in /dev/shm (operator-flagged 2026-06-07). Replace the ignored
`shm_size: 512m` with a tmpfs mount on /dev/shm (size 512MB), which Swarm AND
plain Compose both honor. Requires a stack redeploy to take effect.

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

docker-compose.yml

@@ -21,17 +21,23 @@ 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}
       POSTGRES_DB: ${DB_NAME:-fabledcurator}
     volumes:
       - postgres_data:/var/lib/postgresql/data
+      # Docker's default /dev/shm is 64MB; VACUUM (ANALYZE) + parallel queries
+      # allocate a POSIX dynamic-shared-memory segment in /dev/shm 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).
+      # NOTE: a `shm_size:` key is SILENTLY IGNORED under Docker Swarm
+      # (`docker stack deploy` — the prod deployment here), so size /dev/shm via
+      # a tmpfs mount instead — honored by both Swarm and plain Compose.
+      - type: tmpfs
+        target: /dev/shm
+        tmpfs:
+          size: 536870912  # 512 MB
     healthcheck:
       test: ["CMD-SHELL", "pg_isready -U ${DB_USER:-fabledcurator}"]
       interval: 10s