Add global presence heartbeat and online tracking

app/db.py

@@ -66,4 +66,13 @@ def init_db(app):
         );
         """
         )
-        db.commit()
+
+        # Lightweight migration support for existing databases.
+        user_columns = {
+            row["name"]
+            for row in db.execute("PRAGMA table_info(users)").fetchall()
+        }
+        if "last_seen_at" not in user_columns:
+            db.execute("ALTER TABLE users ADD COLUMN last_seen_at TIMESTAMP")
+
+        db.commit()