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/models.py

@@ -1,3 +1,4 @@
+from django.conf import settings
 from django.db import models
 
 
@@ -31,7 +32,8 @@ class Artifact(models.Model):
     ]
 
     id = models.CharField(max_length=8, primary_key=True)
-    persona = models.CharField(max_length=80, db_index=True)
+    owner = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE, related_name="artifacts")
+    source = models.CharField(max_length=80, db_index=True)
     title = models.CharField(max_length=200)
     kind = models.CharField(max_length=20, choices=KIND_CHOICES)
     theme = models.CharField(max_length=40, blank=True)
@@ -45,9 +47,22 @@ class Artifact(models.Model):
     source_name = models.CharField(max_length=120, blank=True)
     source_id = models.CharField(max_length=120, blank=True)
     metadata = models.JSONField(default=dict, blank=True)
+    read_at = models.DateTimeField(null=True, blank=True)
+
+    @property
+    def is_unread(self):
+        return self.read_at is None
 
     class Meta:
         ordering = ["-created_at"]
 
     def __str__(self):
-        return f"{self.title} ({self.persona})"
+        return f"{self.title} ({self.source})"
+
+
+class TagColor(models.Model):
+    name = models.CharField(max_length=40, primary_key=True)
+    color = models.CharField(max_length=20)
+
+    def __str__(self):
+        return f"{self.name} ({self.color})"