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/storage.py

@@ -6,11 +6,13 @@ usage is a cheap SUM rather than a stat of every file.
 """
 
 import shutil
+from datetime import timedelta
 
 from django.conf import settings
 from django.db.models import Sum
+from django.utils import timezone
 
-from .models import Artifact, UserProfile
+from .models import Artifact, Asset, UserProfile
 
 
 class QuotaExceeded(Exception):
@@ -32,19 +34,38 @@ def artifact_disk_size(relative_path, kind):
 
 
 def delete_artifact_files(artifact):
-    """Remove an artifact's files from disk (primary + markdown sidecar)."""
+    """Remove an artifact's files from disk (primary, markdown sidecar, assets).
+
+    Assets are cascade-deleted from the DB by the FK, but their files must be
+    removed here explicitly.
+    """
     root = settings.ARTIFACTS_ROOT
     primary = root / artifact.path
     primary.unlink(missing_ok=True)
     if artifact.kind == Artifact.MARKDOWN:
         primary.with_suffix(".md").unlink(missing_ok=True)
+    for asset in artifact.assets.all():
+        delete_asset_file(asset)
+
+
+def delete_asset_file(asset):
+    (settings.ARTIFACTS_ROOT / asset.path).unlink(missing_ok=True)
+
+
+def artifact_total_bytes(artifact):
+    """An artifact's own bytes plus those of the assets embedded in it."""
+    asset_bytes = artifact.assets.aggregate(total=Sum("size_bytes")).get("total") or 0
+    return artifact.size_bytes + asset_bytes
 
 
 def user_usage_bytes(user):
-    return (
-        Artifact.objects.filter(owner=user).aggregate(total=Sum("size_bytes")).get("total")
-        or 0
+    artifact_bytes = (
+        Artifact.objects.filter(owner=user).aggregate(total=Sum("size_bytes")).get("total") or 0
     )
+    asset_bytes = (
+        Asset.objects.filter(owner=user).aggregate(total=Sum("size_bytes")).get("total") or 0
+    )
+    return artifact_bytes + asset_bytes
 
 
 def user_quota_bytes(user):
@@ -73,7 +94,8 @@ def make_room_for(user, incoming_bytes):
     for artifact in unkept:
         if used + incoming_bytes <= quota:
             break
-        used -= artifact.size_bytes
+        # Freeing an artifact also frees the assets embedded in it.
+        used -= artifact_total_bytes(artifact)
         delete_artifact_files(artifact)
         artifact.delete()
         evicted.append(artifact)
@@ -83,6 +105,27 @@ def make_room_for(user, incoming_bytes):
     return evicted
 
 
+ASSET_ORPHAN_GRACE = timedelta(hours=24)
+
+
+def sweep_orphan_assets(user=None):
+    """Delete assets uploaded but never referenced by an artifact.
+
+    Runs opportunistically on upload. The grace period gives an agent time to
+    upload images and then post the artifact that references them.
+    """
+    cutoff = timezone.now() - ASSET_ORPHAN_GRACE
+    orphans = Asset.objects.filter(artifact__isnull=True, created_at__lt=cutoff)
+    if user is not None:
+        orphans = orphans.filter(owner=user)
+    count = 0
+    for asset in orphans:
+        delete_asset_file(asset)
+        asset.delete()
+        count += 1
+    return count
+
+
 def disk_free_bytes():
     """Free space on the volume that holds the artifact store."""
     return shutil.disk_usage(settings.ARTIFACTS_ROOT).free