Add multi-user auth, settings area, and unified label feed

Turns Daybook into a multi-user tool and reworks the feed UI:

- Multi-user: Django session login (username/password + remember-me),
  per-user personal API tokens (DRF authtoken), whole app gated behind
  login. Each artifact has an owner; feeds are private per user.
- Rename persona -> source (free-text label for whatever posted an
  artifact: agent-profile, script, or person). source + tags form one
  unified label system; single label filter matches tags, source has
  its own filter/column/sidebar.
- Settings area (gear/user menu, top-right): Account (view/regenerate
  token) and Demo Data (in-app button seeding one artifact per kind).
  User account management via Django admin (superusers only).
- Feed rework: rolling 7-day window with sticky inline date separators
  and infinite scroll; left sidebar is now a Sources list with per-source
  unread counts; artifact type shown as text after title; source in its
  own column.
- Unread tracking: read_at on Artifact, marked read when previewed,
  bold titles for unread, sidebar unread badges, old banner removed.
- Tag colours: global shared registry, hash-based default with optional
  per-tag override via tag_colors on POST/PATCH.
- Ingest: token resolves owner; same source name under two users' tokens
  stays isolated. daybook_post.py renamed persona->source, requires a
  real token, --check accepts a bearer token. Docs updated to match.

Migrations 0004-0008 rename persona, add owner (nullable -> backfill to
first superuser -> required), and add read_at. Fresh installs no-op the
backfill.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/artifacts/settings_base.html

@@ -0,0 +1,28 @@
+<!doctype html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1">
+  <title>Settings — AgentFeed</title>
+  <link rel="stylesheet" href="/static/daybook.css">
+</head>
+<body>
+  {% include "artifacts/_topbar.html" %}
+
+  <main class="settings-layout">
+    <nav class="settings-nav">
+      <a class="settings-nav-back" href="{% url 'feed' %}">&larr; Back to Feed</a>
+      <a class="{% if active_section == 'account' %}selected{% endif %}" href="{% url 'settings-account' %}">Account</a>
+      <a class="{% if active_section == 'demo-data' %}selected{% endif %}" href="{% url 'settings-demo-data' %}">Demo Data</a>
+    </nav>
+    <section class="settings-panel">
+      {% if messages %}
+        <ul class="settings-messages">
+          {% for message in messages %}<li>{{ message }}</li>{% endfor %}
+        </ul>
+      {% endif %}
+      {% block settings_content %}{% endblock %}
+    </section>
+  </main>
+</body>
+</html>