mAi: #105 - docker-compose: add PALIAD_EXPORT_DIR + paliad_exports volume

Slice A Backup Mode (m/paliad#77) needs PALIAD_EXPORT_DIR set on the web
container, otherwise /admin/backups returns 503. Declare it via env
interpolation with a sensible compose-level default and mount a named
volume so backups persist across container restarts.

- env: PALIAD_EXPORT_DIR=${PALIAD_EXPORT_DIR:-/var/lib/paliad/exports}
- volume mount: paliad_exports:/var/lib/paliad/exports
- top-level: declare paliad_exports volume (default driver)

Verified: `docker compose config` resolves cleanly,
`go build ./... && go test ./internal/...` clean,
`cd frontend && bun run build` clean (no code change).

Closes m/paliad#105 once Dokploy auto-redeploys.

docker-compose.yml

@@ -42,5 +42,14 @@ services:
       - AICHAT_URL=${AICHAT_URL:-}
       - AICHAT_TOKEN=${AICHAT_TOKEN:-}
       - AICHAT_PERSONA=${AICHAT_PERSONA:-paliadin}
+      # Backup Mode (m/paliad#77 Slice A). Local-disk export target; the
+      # paliad_exports named volume below persists it across container
+      # restarts. Unset → /admin/backups returns 503 (BackupService gate).
+      - PALIAD_EXPORT_DIR=${PALIAD_EXPORT_DIR:-/var/lib/paliad/exports}
       # - ANTHROPIC_API_KEY=${ANTHROPIC_API_KEY}  # Phase H (AI Frist-Extraktion), currently deferred
+    volumes:
+      - paliad_exports:/var/lib/paliad/exports
     restart: unless-stopped
+
+volumes:
+  paliad_exports: