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/database/repository_debug_test.go

@@ -147,7 +147,7 @@ func TestOrphanedFileCleanupDebug(t *testing.T) {
 	t.Logf("Files count after cleanup: %d", count)
 
 	// List remaining files
-	files, err := repos.Files.ListByPrefix(ctx, "/")
+	files, err := repos.Files.ListUnderPath(ctx, "/")
 	if err != nil {
 		t.Fatal(err)
 	}