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.

tests/e2e/profile/index.py

@@ -1,5 +1,6 @@
 # retoor <retoor@molodetz.nl>
 
+import re
 from tests.conftest import BASE_URL
 FOLLOW = "form[action='/follow/bob_test'] button"
 UNFOLLOW = "form[action='/follow/unfollow/bob_test'] button"
@@ -7,6 +8,25 @@ from playwright.sync_api import expect
 from tests.e2e.post import create_post
 
 
+def test_profile_shows_online_presence(alice):
+    page, user = alice
+    page.goto(
+        f"{BASE_URL}/profile/{user['username']}", wait_until="domcontentloaded"
+    )
+    presence = page.locator(".profile-presence")
+    expect(presence).to_be_visible()
+    expect(presence).to_have_class(re.compile(r"\bonline\b"))
+    expect(presence).to_have_text("online")
+
+
+def test_nav_avatar_shows_online_presence_dot(alice):
+    page, _ = alice
+    page.goto(f"{BASE_URL}/feed", wait_until="domcontentloaded")
+    nav_dot = page.locator(".topnav-user .presence-dot")
+    expect(nav_dot).to_be_visible()
+    expect(nav_dot).to_have_class(re.compile(r"\bonline\b"))
+
+
 def _post_comment(page, body):
     textarea = page.locator(".comment-form textarea[name='content']")
     textarea.fill(body)