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

internal/database/files.go

@@ -266,6 +266,56 @@ func (r *FileRepository) ListByPrefix(
 	return files, rows.Err()
 }
 
+// ListIDsWithChunksNotInUploadedBlobs returns the IDs of the files whose
+// path starts with prefix and that list at least one chunk held by no
+// blob whose upload has completed (uploaded_ts set). Such a file's
+// metadata can still match the file on disk while part of its data is
+// not in remote storage, so a backup must not treat it as unchanged.
+func (r *FileRepository) ListIDsWithChunksNotInUploadedBlobs(
+	ctx context.Context, prefix string,
+) ([]types.FileID, error) {
+	query := `
+		SELECT DISTINCT f.id
+		FROM files f
+		JOIN file_chunks fc ON fc.file_id = f.id
+		WHERE f.path LIKE ? || '%'
+		  AND NOT EXISTS (
+			SELECT 1
+			FROM blob_chunks bc
+			JOIN blobs b ON bc.blob_id = b.id
+			WHERE bc.chunk_hash = fc.chunk_hash
+			  AND b.uploaded_ts IS NOT NULL
+		  )
+	`
+
+	rows, err := r.db.conn.QueryContext(ctx, query, prefix)
+	if err != nil {
+		return nil, fmt.Errorf("querying files: %w", err)
+	}
+
+	defer func() {
+		err := rows.Close()
+		if err != nil {
+			Fatalf("failed to close rows: %v", err)
+		}
+	}()
+
+	var ids []types.FileID
+
+	for rows.Next() {
+		var id types.FileID
+
+		err := rows.Scan(&id)
+		if err != nil {
+			return nil, fmt.Errorf("scanning file ID: %w", err)
+		}
+
+		ids = append(ids, id)
+	}
+
+	return ids, rows.Err()
+}
+
 // ListAll returns all files in the database
 func (r *FileRepository) ListAll(ctx context.Context) ([]*File, error) {
 	query := `