Add global presence heartbeat and online tracking

app/routes/presence.py

@@ -0,0 +1,24 @@
+from flask import Blueprint, jsonify
+from flask_login import login_required, current_user
+from app.db import get_db
+
+presence_bp = Blueprint("presence", __name__)
+
+def _mark_current_user_online() -> None:
+    db = get_db()
+    db.execute(
+        "UPDATE users SET last_seen_at = CURRENT_TIMESTAMP WHERE id = ?",
+        (current_user.id,),
+    )
+    db.commit()
+
+
+@presence_bp.route("/api/presence/ping", methods=["POST"])
+@login_required
+def presence_ping():
+    if not current_user.is_authenticated: # should not happen due to @login_required, but just in case
+        return ("", 204)
+
+    _mark_current_user_online()
+    return jsonify({"status": "ok"})
+