v0.7.1: in-app password change, custom labels, always-on folders

- Settings -> Account: change your own password (current + new), session
  preserved via update_session_auth_hash.
- Custom labels: per-user palette in Settings -> Labels; apply/remove from a
  message's ... menu (menu stays open, checkmark = applied). Labels are unified
  with tags; chip colours sync to the global TagColor. New Label model +
  migration 0017; POST /a/{id}/label (session, owner-scoped, palette-validated).
- Sidebar Keep/Deleted folders are now always shown; count badge only when
  non-empty. Updates the stale 'hide when empty' note in CLAUDE.md.

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

artifacts/migrations/0017_label.py

@@ -0,0 +1,29 @@
+# Generated by Django 5.2.15 on 2026-09-08 13:46
+
+import django.db.models.deletion
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('artifacts', '0016_artifact_deleted_at'),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Label',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('name', models.CharField(max_length=40)),
+                ('color', models.CharField(max_length=20)),
+                ('user', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='labels', to=settings.AUTH_USER_MODEL)),
+            ],
+            options={
+                'ordering': ['name'],
+                'unique_together': {('user', 'name')},
+            },
+        ),
+    ]