fix(dstack): persist SQL and DuckDB data

[skgbafa]

Summary

• mount a persistent tinycloud-data Docker volume at /data in the dstack Postgres+S3 compose
• set TINYCLOUD_STORAGE_DATADIR=/data so embedded SQL and DuckDB files persist across container updates/restarts
• add a focused compose regression check for the dstack persistence shape

Verification

• git diff --check HEAD~1..HEAD
• sh -n scripts/check-dstack-postgres-persistence.sh
• ./scripts/check-dstack-postgres-persistence.sh

Linear: TC-1387

Notes

• No production deploy performed.
• This does not recover any data already lost from a previous ephemeral container replacement.

[skgbafa]

Closing this PR as superseded by the protocol-level storage issue.

The Docker volume change is only an emergency mitigation; it does not satisfy the TinyCloud storage invariant that SQL/DuckDB user data must persist through the configured TinyCloud storage backend rather than an implicit container-local filesystem.

Linear: https://linear.app/tinycloudlabs/issue/TC-1388/persist-sql-and-duckdb-through-configured-tinycloud-storage-backend