Add self-update, including in Docker

Sandbar can install new releases itself using Reef's selfupdate, automatically with
SANDBAR_AUTO_UPDATE=1 or from a button on the Account page. In a container the new binary is
written to <data>/bin and takes over from the image at start, until the image catches up.
Tags now publish a Gitea release with archives and SHA256SUMS as well as the image.

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

.env.example

@@ -23,6 +23,13 @@ SANDBAR_MASTER_KEY=
 # Leave empty and /metrics does not exist.
 #SANDBAR_METRICS_TOKEN=
 
+# Install new releases by itself: checks every few hours, then downloads, verifies and restarts.
+# Works in Docker too (the new binary is kept in <data>/bin). Off by default.
+#SANDBAR_AUTO_UPDATE=1
+# Where releases come from, if not this project on git.veillax.com.
+#SANDBAR_UPDATE_SERVER=https://git.veillax.com
+#SANDBAR_UPDATE_REPO=Veillax/sandbar
+
 # How long topic messages and monitor results are kept, in days.
 #SANDBAR_MESSAGE_DAYS=30
 #SANDBAR_CHECK_DAYS=30