Fix migration of foreign keys

The tests currently fail, due to migration m008_reply_to_id_foreign_key
failing. I suspect this is related to a recent change in squlite-utils
[0].

This PR avoids that by explictly dropping the foreign key before
renaming the table, then adding the foreign key back.

Fixes simonw#162

[0] simonw/sqlite-utils#577

llm/migrations.py

@@ -130,8 +130,10 @@ def m007_finish_logs_table(db):
         drop={"debug"},
         rename={"timestamp_utc": "datetime_utc"},
     )
+    db["log"].transform(drop_foreign_keys=("chat_id",))
     with db.conn:
         db.execute("alter table log rename to logs")
+    db["logs"].add_foreign_key("chat_id", "logs", "id")
 
 
 @migration