Show database migration progress during service upgrades

Context

Database migrations can make a service upgrade appear stalled. A migration tested while preparing #38367 took roughly 37 seconds on an unusually large 8.8 GB database even after batching reduced its memory and runtime costs.

Idea

Let migrations optionally report bounded progress and surface it in the service-upgrade TUI. For migrations that know their total work, show completed/total progress; otherwise show the active migration and an indeterminate indicator.

This is a future UX improvement rather than a requirement for #38367.

Considerations

• Keep migrations usable outside the interactive TUI.
• Avoid requiring a full-table count merely to calculate progress.
• Preserve useful noninteractive/log output.
• Ensure progress reporting does not materially slow the migration.

Requested by: @kitlangton (Kit via Slack)