feat(setup): show running build version in the Setup tab bar

CI already computes <branch>-<short-sha> for image tags but never
surfaced it anywhere reachable from the app, so there was no way to
tell what's actually deployed on a booth without cross-referencing
komodo/resources.toml's TAG by hand.

Thread it through: CI passes BUILD_VERSION as a Docker build-arg,
the Dockerfile captures it as a runtime env var, GET /api/version
(gated by the existing site:read permission) exposes it, and the
Setup page's tab bar shows it right-aligned, muted, absent entirely
on a local/dev build with no CI-supplied value.

Claude-Session: https://claude.ai/code/session_01FWncR69HgGPuei1dLrW3cU

apps/server/src/routes/site.ts

@@ -78,6 +78,15 @@ export async function siteRoutes(app: FastifyInstance, db: Db, eventLog?: EventL
   // Live occupancy: cars inside, capacity, free, full. Any signed-in role.
   app.get("/api/occupancy", { preHandler: readGuard }, async () => getOccupancy(db));
 
+  // Running build version ("<branch>-<short-sha>", matching the Komodo Stack's TAG in
+  // komodo/resources.toml) — baked in at image build time (apps/server/Dockerfile
+  // BUILD_VERSION ARG), read here from the running process env. null on a local/dev
+  // build with no CI-supplied value. Purely informational (Setup nav display); not
+  // site config, so it isn't stored in site_config.
+  app.get("/api/version", { preHandler: readGuard }, async () => ({
+    buildVersion: process.env.BUILD_VERSION?.trim() || null,
+  }));
+
   // Read site config (capacity + park metadata).
   app.get("/api/site-config", { preHandler: readGuard }, async () => {
     const row = db.select().from(siteConfig).where(eq(siteConfig.id, 1)).get();