User-level Postgres storage rollup.

Reports the bytes the user's data occupies on disk — each channel's own ch_* database — and a count of the databases that number was measured from. A user without any app owns no channel row, so the zero-usage shape falls out without a probe. Returns 503 when the admin pool / config is unavailable (degraded mode) or when any part of the rollup fails.

The route only reads the control DB, so it runs on run_read_in_thread. It costs one round trip to the control DB, and the Postgres core reads the channels' rows in a second one. The sizes come from the tenant Postgres.