Add image assets embeddable in artifact content

Agents can now put images inside markdown/HTML artifacts instead of only
posting them as standalone artifacts.

- New Asset model: an image uploaded ahead of the artifact that embeds it.
- POST /api/assets uploads an image (image/* only, 50 MB cap) and returns
  a stable URL the agent references normally, e.g. ![chart](url). No custom
  syntax; existing markdown/fragment rendering handles it.
- GET /assets/{id} serves the image, owner-scoped (404 for other users).
- On artifact creation the content is scanned for asset URLs and matching
  assets are attached to it, so the image dies with the artifact it belongs
  to. Uploads never referenced are swept after a 24h grace period.
- Quota: assets count toward the owner's quota at upload time and can
  trigger eviction; evicting an artifact frees its assets too, and usage
  now sums artifacts plus assets.
- daybook_post.py gains --upload-image, which prints the URL to embed.
- Docs and SKILL.md cover the upload-then-reference workflow.

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

artifacts/migrations/0011_add_asset.py

@@ -0,0 +1,31 @@
+# Generated by Django 5.2.15 on 2026-07-24 10:13
+
+import django.db.models.deletion
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('artifacts', '0010_backfill_sizes_and_profiles'),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Asset',
+            fields=[
+                ('id', models.CharField(max_length=12, primary_key=True, serialize=False)),
+                ('path', models.CharField(max_length=500)),
+                ('content_type', models.CharField(max_length=100)),
+                ('size_bytes', models.BigIntegerField(default=0)),
+                ('created_at', models.DateTimeField(auto_now_add=True)),
+                ('artifact', models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE, related_name='assets', to='artifacts.artifact')),
+                ('owner', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='assets', to=settings.AUTH_USER_MODEL)),
+            ],
+            options={
+                'ordering': ['-created_at'],
+            },
+        ),
+    ]