Connect Superset to DuckDB read-only

[cnstlungu]

Testing whether read-only access removes the uid dependency. The CI chmod workaround is removed in this branch so the run actually exercises the fix.

[cnstlungu]

Closing: disproven. A direct duckdb.connect(read_only=True) inside the container opens the 644 file owned by another uid just fine, but access_mode=read_only never reaches DuckDB — superset import-dashboards defines the connection from the URI embedded in dashboard.zip, overriding what set_database_uri stored, and connects read-write. Superset therefore needs write permission on the warehouse file; the fix has to grant it.