Fix asyncpg datetime type error in create_asset and create_activity

asyncpg requires datetime objects, not ISO strings. Added _parse_timestamp
helper to convert string timestamps to datetime objects.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

db.py

@@ -14,6 +14,22 @@ from uuid import UUID
 import asyncpg
 
 # Connection pool (initialized on startup)
+
+
+def _parse_timestamp(ts) -> datetime:
+    """Parse a timestamp string or datetime to datetime object."""
+    if ts is None:
+        return datetime.now(timezone.utc)
+    if isinstance(ts, datetime):
+        return ts
+    # Parse ISO format string
+    if isinstance(ts, str):
+        if ts.endswith('Z'):
+            ts = ts[:-1] + '+00:00'
+        return datetime.fromisoformat(ts)
+    return datetime.now(timezone.utc)
+
+
 _pool: Optional[asyncpg.Pool] = None
 
 # Configuration from environment
@@ -246,7 +262,7 @@ async def create_asset(asset: dict) -> dict:
             asset.get("description"),
             json.dumps(asset.get("origin")) if asset.get("origin") else None,
             asset["owner"],
-            asset.get("created_at") or datetime.now(timezone.utc).isoformat()
+            _parse_timestamp(asset.get("created_at"))
         )
         return _parse_asset_row(row)
 
@@ -376,7 +392,7 @@ async def create_activity(activity: dict) -> dict:
             activity["activity_type"],
             activity["actor_id"],
             json.dumps(activity["object_data"]),
-            activity["published"],
+            _parse_timestamp(activity["published"]),
             json.dumps(activity.get("signature")) if activity.get("signature") else None
         )
         return _parse_activity_row(row)