Add per-user disk quotas, auto-eviction, and Keep flag

Bounds each user's disk footprint and makes throwaway artifacts self-clean:

- Per-user quota (default 1 GB) stored on a new UserProfile model,
  adjustable per user in Django admin. Artifact.size_bytes records each
  artifact's on-disk footprint (primary file + markdown sidecar) at
  ingest so usage is a cheap SUM.
- Automatic eviction: on each post, oldest unkept artifacts are deleted
  (files and rows) until the new one fits under quota. If the quota is
  full of kept artifacts, the post is rejected with HTTP 413 rather than
  evicting anything protected.
- Keep flag (Artifact.is_kept): a Keep button in the preview control bar
  toggles protection via POST /a/{id}/keep; kept items are never evicted.
  Kept rows show a green "K" badge in the listing.
- Details panel: a Details toggle in the control bar expands a panel
  under it with full source, type, created time, size, and tags.
- Settings -> Storage: usage bar (green/amber/red), used/quota summary,
  artifact and kept counts; superusers also see total allocated quota vs
  free disk with an oversubscription warning.
- Type indicator in the listing is now a small grey pill instead of
  plain text.

Storage accounting/eviction logic lives in artifacts/storage.py.
Migrations 0009 (fields + UserProfile) and 0010 (backfill sizes and
create profiles for existing users). Fresh installs no-op the backfill.

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

artifacts/models.py

@@ -48,6 +48,8 @@ class Artifact(models.Model):
     source_id = models.CharField(max_length=120, blank=True)
     metadata = models.JSONField(default=dict, blank=True)
     read_at = models.DateTimeField(null=True, blank=True)
+    is_kept = models.BooleanField(default=False)
+    size_bytes = models.BigIntegerField(default=0)
 
     @property
     def is_unread(self):
@@ -66,3 +68,13 @@ class TagColor(models.Model):
 
     def __str__(self):
         return f"{self.name} ({self.color})"
+
+
+class UserProfile(models.Model):
+    DEFAULT_QUOTA_BYTES = 1024 * 1024 * 1024  # 1 GB
+
+    user = models.OneToOneField(settings.AUTH_USER_MODEL, on_delete=models.CASCADE, related_name="profile")
+    quota_bytes = models.BigIntegerField(default=DEFAULT_QUOTA_BYTES)
+
+    def __str__(self):
+        return f"{self.user} quota={self.quota_bytes}"