A Content Editor tab left open across a deploy (or a save from another tab)
held the pre-deploy content; one Save would silently overwrite the newer
file. The editor page now embeds a SHA-256 fingerprint of the file content
at load time, /save requires it back in X-Content-Hash and compares against
the current file: mismatch (or a missing header) answers 409 with an
explanatory message and writes nothing. The client offers a reload on 409.
Integration test covers: matching hash saves, stale hash rejected with the
file untouched, missing hash rejected, retry with the fresh hash succeeds.
Closes MITHOME-61
