Ignore the server-minted install-id file

In local dev the server writes its per-install UUID to the repo root
(on the box it lives at /data/install-id), so it kept showing up as
untracked. It's machine-local runtime identity — never commit it.

.gitignore

@@ -19,6 +19,9 @@ image.tar
 
 # Runtime / user data — must never be committed
 history/
+# Per-install identity (UUID) the server mints on first boot. On the box it
+# lives at /data/install-id; in local dev the server writes it to the repo root.
+/install-id
 cookies.txt
 *.txt.bak
 youtube-summarizer-library-export*.json