feat: show live build version in web UI

- /api/version returns version, commitSha, buildDate, trashRetentionDays
- CI passes COMMIT_SHA (full), BUILD_DATE and VERSION (git tag) as
  docker build args; Dockerfile bakes them as env
- sidebar footer shows version/sha linked to the Gitea commit
- admin System tab shows version, commit link, build date, trash retention

artifacts/api-server/src/routes/health.ts

@@ -8,4 +8,14 @@ router.get("/healthz", (_req, res) => {
   res.json(data);
 });
 
+router.get("/version", (_req, res) => {
+  const retention = Number(process.env["TRASH_RETENTION_DAYS"] ?? "0");
+  res.json({
+    version: process.env["VERSION"] || "dev",
+    commitSha: process.env["COMMIT_SHA"] || null,
+    buildDate: process.env["BUILD_DATE"] || null,
+    trashRetentionDays: Number.isFinite(retention) ? retention : 0,
+  });
+});
+
 export default router;