feat: burn-after-read and expiry cleaner

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/services/cleaner.py

@@ -0,0 +1,35 @@
+"""Periodic cleanup of expired and burned items."""
+
+from __future__ import annotations
+
+from datetime import datetime
+
+from sqlalchemy import or_, select
+from sqlalchemy.orm import Session
+
+from app.models import Item
+from app.services import items as items_service
+
+
+def run_once(db: Session, *, now: datetime | None = None) -> int:
+    """Delete expired rows (+ files) and burned rows (leftover files).
+
+    Returns the number of item rows removed.
+    """
+    now = now or datetime.utcnow()
+    rows = list(
+        db.scalars(
+            select(Item).where(
+                or_(
+                    Item.burned.is_(True),
+                    (Item.expires_at.is_not(None) & (Item.expires_at <= now)),
+                )
+            )
+        ).all()
+    )
+    for item in rows:
+        items_service.delete_item_file(item)
+        db.delete(item)
+    if rows:
+        db.commit()
+    return len(rows)