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>

daybook_project/settings.py

@@ -17,6 +17,7 @@ INSTALLED_APPS = [
     "django.contrib.messages",
     "django.contrib.staticfiles",
     "rest_framework",
+    "rest_framework.authtoken",
     "artifacts",
 ]
 
@@ -59,6 +60,9 @@ DATABASES = {
 
 AUTH_PASSWORD_VALIDATORS = []
 
+LOGIN_URL = "login"
+LOGIN_REDIRECT_URL = "feed"
+
 LANGUAGE_CODE = "en-us"
 TIME_ZONE = "UTC"
 USE_I18N = True
@@ -76,4 +80,3 @@ STORAGES = {
 DEFAULT_AUTO_FIELD = "django.db.models.BigAutoField"
 
 ARTIFACTS_ROOT = Path(os.environ.get("ARTIFACTS_ROOT", DATA_DIR / "artifacts_store"))
-DAYBOOK_API_TOKEN = os.environ.get("DAYBOOK_API_TOKEN", "dev-token")