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/schema.py

@@ -36,6 +36,7 @@ def init_db():
     _index(db, "users", "idx_users_email", ["email"])
     _index(db, "users", "idx_users_api_key", ["api_key"])
     _index(db, "users", "idx_users_role", ["role", "created_at"])
+    _index(db, "users", "idx_users_last_seen", ["last_seen"])
     _index(db, "posts", "idx_posts_user_uid", ["user_uid"])
     _index(db, "posts", "idx_posts_created_at", ["created_at"])
     _index(db, "posts", "idx_posts_topic", ["topic"])
@@ -1117,6 +1118,8 @@ def backfill_api_keys() -> int:
         users.create_column_by_example("timezone", "")
     if not users.has_column("avatar_seed"):
         users.create_column_by_example("avatar_seed", "")
+    if not users.has_column("last_seen"):
+        users.create_column_by_example("last_seen", "")
     with db:
         db.query(
             "UPDATE users SET ai_modifier_enabled = 1 WHERE ai_modifier_enabled IS NULL"