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>

artifacts/migrations/0006_backfill_owner.py

@@ -0,0 +1,27 @@
+from django.db import migrations
+
+
+def backfill_owner(apps, schema_editor):
+    User = apps.get_model("auth", "User")
+    Artifact = apps.get_model("artifacts", "Artifact")
+
+    first_superuser = User.objects.filter(is_superuser=True).order_by("date_joined").first()
+    if first_superuser is None:
+        return
+
+    Artifact.objects.filter(owner__isnull=True).update(owner=first_superuser)
+
+
+def noop_reverse(apps, schema_editor):
+    pass
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("artifacts", "0005_add_owner_nullable"),
+    ]
+
+    operations = [
+        migrations.RunPython(backfill_owner, noop_reverse),
+    ]