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/schemas/content.py

@@ -19,6 +19,7 @@ class UserOut(_Out):
     xp: Optional[int] = None
     stars: Optional[int] = None
     created_at: Optional[str] = None
+    last_seen: Optional[str] = None
 
 
 class AttachmentOut(_Out):

devplacepy/schemas/listings.py

@@ -113,6 +113,7 @@ class FeedOut(_Out):
     total_gists: Optional[int] = None
     top_authors: list[UserOut] = []
     daily_topic: Optional[Any] = None
+    online_users: list[UserOut] = []
 
 
 class PostDetailOut(_Out):

devplacepy/schemas/profile.py

@@ -40,6 +40,7 @@ class ProfileOut(_Out):
     is_blocked: bool = False
     is_muted: bool = False
     is_owner: bool = False
+    profile_online: bool = False
     can_view_api_key: bool = False
     api_key: Optional[str] = None
     ai_correction_enabled: bool = False