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/vaultik/integration_test.go

@@ -249,7 +249,7 @@ func verifyEndToEndBackupState(
 	assert.Positive(t, blobUploads, "Should upload at least one blob")
 
 	// Verify files in database
-	files, err := repos.Files.ListByPrefix(ctx, "/home/user")
+	files, err := repos.Files.ListUnderPath(ctx, "/home/user")
 	require.NoError(t, err)
 	// Count only regular files (not directories)
 	regularFiles := 0