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.

.gitea/workflows/ci.yml

@@ -54,6 +54,14 @@ jobs:
             fetch --depth 1 origin "${{ gitea.sha }}"
           git checkout -q FETCH_HEAD
 
+      # Read by app.rb at boot and shown (as a hover tooltip on the topbar
+      # brand) so it's possible to tell which commit is actually live
+      # without digging through `tea actions runs list` — written here,
+      # before .git gets dropped below, since that's otherwise the only
+      # place the commit is recorded.
+      - name: Write REVISION
+        run: git rev-parse --short HEAD > REVISION
+
       # Clears out everything in the target except data/ (uploaded GPX
       # files, offline.pmtiles) and copies the fresh checkout over, dropping
       # .git afterwards. Dotfiles like .env (deployment secrets) are left