Fix two CI jobs left behind by the CIDB connection-secret rename

`Settings.SECRET_CI_DB_URL`, `Settings.SECRET_CI_DB_USER` and
`Settings.SECRET_CI_DB_PASSWORD` were replaced by a single
`Settings.SECRET_CI_DB_CONNECTION` (a JSON blob with `url`, `user` and
`password`) plus the `CIDB.from_connection_secret` helper. Two callers
were not migrated and now die on startup with

    AttributeError: '_Settings' object has no attribute 'SECRET_CI_DB_URL'

- `Revert CI regressions` in the `Hourly` workflow has failed on every
  run since 2026-08-31 09:05 UTC. It crashes in `connect` before it
  reads anything, so no master failure has been investigated and no
  regression reverted since then, and nothing has been written to
  `checks_investigated` since 2026-08-31 08:10:59 UTC.
- `Collect Test Duration Statistics` in the `NightlyStatistics`
  workflow has failed the same way since 2026-09-01 05:23 UTC.

The secret itself needs no change: `clickhouse-test-stat-connection` is
already declared in `SECRETS`, which both workflows use.

Checked every other reference: a sweep of all `Settings.<NAME>` uses
outside `ci/praktika` against the fields of the current `_Settings` and
`ci/settings/settings.py` finds no other stragglers, and no other
scheduled workflow or CIDB-reported check changed from green to red at
that merge.