List the files under a path by path, not by string prefix (closes #223)

FileRepository.ListByPrefix matched with SQL LIKE: a plain string
prefix that ignores ASCII case and treats _ and % as wildcards.
Restoring /home/u/doc also restored doc2, DOC and doc.txt.bak, and a
backup counted the files of a longer sibling path as deleted. It is
now ListUnderPath, which returns the file at the path and every file
whose path starts with the path plus a slash, compared exactly. A
trailing slash is ignored, so "/" still lists every file.

Three tests relied on string-prefix matching and now name a full path
or call ListAll.

ListIDsWithChunksNotInUploadedBlobs keeps its LIKE: it only adds file
IDs the scan never looks up.

Model: opus-5-5

internal/snapshot/scanner.go

@@ -456,14 +456,16 @@ func (s *Scanner) finalizeScanResult(ctx context.Context, result *ScanResult) {
 	result.EndTime = time.Now().UTC()
 }
 
-// loadKnownFiles loads all known files from the database into a map for fast lookup
-// This avoids per-file database queries during the scan phase
+// loadKnownFiles loads the known files at and beneath path from the
+// database into a map for fast lookup. Every loaded file the scan does
+// not find is counted as deleted. This avoids per-file database queries
+// during the scan phase.
 func (s *Scanner) loadKnownFiles(
 	ctx context.Context, path string,
 ) (map[string]*database.File, error) {
-	files, err := s.repos.Files.ListByPrefix(ctx, path)
+	files, err := s.repos.Files.ListUnderPath(ctx, path)
 	if err != nil {
-		return nil, fmt.Errorf("listing files by prefix: %w", err)
+		return nil, fmt.Errorf("listing files under %s: %w", path, err)
 	}
 
 	result := make(map[string]*database.File, len(files))