Show edge and site versions with Update buttons; Update now for Hammerhead itself

Edges and sites report their version and whether auto-update is on. Hammerhead can ask a machine that
has auto-update on to install the newest release, and shows the outcome. Requests carry an ID so a
stale result can't answer a new one.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -73,6 +73,8 @@ Releases are published for x86-64 only; for ARM machines build it yourself (`TAR
 
 Set `HAMMERHEAD_AUTO_UPDATE=1` (or pass `-auto-update`) and Hammerhead checks this server for a newer release every few hours, verifies it against `SHA256SUMS`, checks that the new binary runs, swaps it in and restarts itself. It only follows full releases (never pre-releases), never downgrades, and keeps the old binary if anything looks wrong. `setup.sh` turns it on. It is not available on Windows, and it is for the systemd and plain-binary installs: with Docker, pull a new image instead.
 
+Edges and sites report their version, so Server → Edges and Sites show which release each one runs and flag a newer one. An **Update** button appears for any machine that runs with auto-update on (`BLACKTIP_AUTO_UPDATE=1` / `BAMBOO_AUTO_UPDATE=1`): it asks that machine to install the newest release now, and the result is shown in the table. Under Server → Settings, **Check for updates** does the same lookup for Hammerhead itself, with an **Update now** button when auto-update is on. A machine without auto-update never installs anything on request.
+
 **With Docker**
 
 ```