Show the deployed commit as a hover tooltip on the topbar brand

ASSET_VERSION only proves *a* deploy happened recently, not *which*
commit -- not enough to answer "did my push actually deploy, or am I
looking at a stale browser cache" (today's actual answer: neither --
deploy was current, a GeoJSON cache predating a code change was just
never invalidated, since it only checks the source .gpx's mtime).

The CI deploy job now writes REVISION (short SHA) before dropping
.git; app.rb reads it at boot, falling back to "dev" locally/in CI's
own check job where the file never exists.

.gitignore

@@ -29,3 +29,7 @@
 # Deployment secrets (SESSION_SECRET, Gitea OAuth2 client id/secret) — lives
 # only on the server, see docs/deployment.adoc
 .env
+
+# Written by the CI deploy job with the commit it deployed — never
+# committed itself, see app.rb's REVISION
+/REVISION