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/static/css/feed.css

@@ -277,6 +277,35 @@
     font-weight: 600;
 }
 
+.online-count {
+    display: inline-block;
+    min-width: 1.25rem;
+    padding: 0 0.375rem;
+    border-radius: 999px;
+    background: var(--success, #2f9e44);
+    color: var(--white);
+    font-size: 0.6875rem;
+    text-align: center;
+    vertical-align: middle;
+}
+
+.online-users-list {
+    display: flex;
+    flex-wrap: wrap;
+    gap: 0.5rem;
+    margin-top: 0.5rem;
+}
+
+.online-user {
+    display: inline-flex;
+    flex-shrink: 0;
+}
+
+.online-empty {
+    color: var(--text-muted);
+    font-size: 0.8125rem;
+}
+
 .community-stats {
     background: var(--bg-card);
     border: 1px solid var(--border);