feat: add online presence tracking with last_seen column and configurable timeout

Add `last_seen` column to users table with index, implement `set_last_seen` and `get_online_users` database functions, expose presence config env vars (`PRESENCE_TIMEOUT_SECONDS`, `PRESENCE_ONLINE_LIMIT`, `PRESENCE_ONLINE_MARGIN_SECONDS`), include `last_seen` in follow list responses, and update profile docs to mention online indicator.

devplacepy/database/users.py

@@ -47,6 +47,30 @@ def set_user_timezone(user_uid: str, tz_name: str) -> None:
     users.update({"uid": user_uid, "timezone": tz_name}, ["uid"])
 
 
+def set_last_seen(user_uid: str, iso: str) -> None:
+    if "users" not in db.tables or not user_uid or not iso:
+        return
+    users = db["users"]
+    if not users.has_column("last_seen"):
+        users.create_column_by_example("last_seen", "")
+    users.update({"uid": user_uid, "last_seen": iso}, ["uid"])
+
+
+def get_online_users(cutoff_iso: str, limit: int = 30) -> list:
+    if "users" not in db.tables:
+        return []
+    users = db["users"]
+    if "last_seen" not in users.columns:
+        return []
+    return list(
+        users.find(
+            last_seen={">=": cutoff_iso},
+            order_by=["username"],
+            _limit=limit,
+        )
+    )
+
+
 def get_primary_admin_uid():
     sync_local_cache("admins", _admins_cache)
     cached = _admins_cache.get("primary")