Re-chunk a known file whose chunks no uploaded blob holds (closes #214)

File rows are shared by every snapshot and updated in place, while a
blob row is deleted once no snapshot references it. Removing the newest
snapshot, or the prune after an interrupted run, could drop the only
blob holding a changed file's current chunks while an older snapshot
kept the file row. The next backup compared metadata only, skipped the
file, and completed a snapshot that could not restore it.

The scanner now loads the IDs of known files that list a chunk no
uploaded blob holds and re-chunks them even when their metadata is
unchanged.

The tests give each backup run its own snapshot name, so the
second-precision snapshot IDs differ without sleeping.

Model: opus-5-5

docs/DATAMODEL.md

@@ -195,6 +195,7 @@ Tracks blob upload metrics.
 1. **Change Detection**
    - `SELECT * FROM files WHERE path = ?` - Get previous file metadata
    - Compare mtime, size, mode to detect changes
+   - Re-chunk a file that lists a chunk no uploaded blob holds, even when its metadata is unchanged
    - Skip unchanged files but still add to `snapshot_files`
 
 2. **Chunk Reuse**