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/_day_groups.html

@@ -0,0 +1,34 @@
+{% for group in day_groups %}
+  <div class="day-group">
+    <div class="day-separator">
+      <span>{{ group.day|date:"D, M j Y" }}{% if group.day == today %} <em class="today-badge">Today</em>{% endif %}</span>
+      <span class="day-separator-count">{{ group.artifacts|length }}</span>
+    </div>
+    {% for artifact in group.artifacts %}
+      <a class="artifact-row {% if selected_artifact.id == artifact.id %}selected{% endif %} {% if artifact.is_unread %}unread{% endif %}"
+         href="?{% if selected_source %}source={{ selected_source|urlencode }}&amp;{% endif %}{% if selected_label %}label={{ selected_label|urlencode }}&amp;{% endif %}{% if selected_kind %}kind={{ selected_kind|urlencode }}&amp;{% endif %}{% if title_query %}q={{ title_query|urlencode }}&amp;{% endif %}artifact={{ artifact.id }}&amp;view={{ selected_view }}"
+         data-artifact-id="{{ artifact.id }}"
+         data-title="{{ artifact.title }}"
+         data-kind="{{ artifact.kind }}"
+         data-rendered-url="{% url 'artifact' artifact.id %}"
+         data-raw-url="{% url 'artifact' artifact.id %}?raw=1">
+        <time class="artifact-time" datetime="{{ artifact.created_at|date:'c' }}">{{ artifact.created_at|date:"H:i" }}</time>
+        <span class="artifact-row-main">
+          <span class="artifact-title">{{ artifact.title }}</span>
+          <span class="artifact-kind-text">{% if artifact.kind == "image" %}image{% elif artifact.kind == "pdf" %}pdf{% elif artifact.kind == "markdown" %}markdown{% elif artifact.kind == "deck" %}deck{% elif artifact.kind == "binary" %}binary{% else %}html{% endif %}</span>
+          {% if artifact.label_chips %}
+            <span class="tag-chips">
+              {% for label in artifact.label_chips %}
+                <span class="tag-chip" style="background:{{ label.color }}">{{ label.name }}</span>
+              {% endfor %}
+            </span>
+          {% endif %}
+        </span>
+        <span class="artifact-source">{{ artifact.source }}</span>
+      </a>
+    {% endfor %}
+  </div>
+{% endfor %}
+{% if has_more %}
+  <div class="feed-more" data-feed-more data-next-before="{{ next_before }}"></div>
+{% endif %}