Reclaim unused SQLite log database pages in the background - #49069

## Why

Deleting logs leaves free SQLite pages available for reuse without shrinking the database file. Reclaim excess free space while preserving a reserve for future writes.

## What changed

- Start a background incremental-vacuum worker with `StateRuntime`, enabling reclamation only for the logs database. Use a per-home file lock to select one worker and wait for it to finish during shutdown.
- Reclaim pages when at least 64 MiB and 25% of the database are free, preserving a reserve of at least 16 MiB or 10% of the database.
- Use short transactions with page and cooperative time budgets, defer work on contention, and reduce batch sizes with exponential backoff when interrupted passes make no progress.
- Record reclamation counts, durations, and reclaimed pages in telemetry.

## Testing

Add a regression test that interrupts a vacuum commit, verifies foreground writes can resume without data loss, then confirms a resumed pass shrinks the database and passes `PRAGMA integrity_check`.

GitOrigin-RevId: 8421f87b0aeef6540eda5690c84216906673c72e