fix: cascade channel_messages on packet delete

[MrAlders0n]

Suggested title: Fix packet retention by adding ON DELETE CASCADE to channel_messages FK

What this PR does

channel_messages.packet_hash references packets with no ON DELETE action, and nothing ever deletes from channel_messages. So any packet carrying a channel message can never age out. Since DeleteOldPackets is a single statement, one blocked row kills the whole delete:

ERROR: update or delete on table "packets" violates foreign key constraint
"channel_messages_packet_hash_fkey" on table "channel_messages" (SQLSTATE 23503)

End result is packet retention silently fails every cleanup tick once your channel traffic is older than your retention window. Easy to miss on a newer deployment since nothing has expired yet. I only found it after lowering retention on a DB that had grown to 83 GB.

Migration 023 recreates the FK with ON DELETE CASCADE, same as packet_observations already does. Channel messages now age out with their packets. Used DROP CONSTRAINT IF EXISTS so it stays idempotent if you already patched this by hand.

Testing notes

Migration only, so sqlc generate is a no-op. Verified on a live deployment, both FKs on packets now show confdeltype = 'c' and the hourly cleanup went from failing every tick to done in a few seconds.

Heads up if your DB has been stuck for a while: the first cleanup after the fix does the entire backlog in one statement and can take a long time. Don't restart the app mid purge, it rolls the whole thing back. And plain DELETE doesn't give space back to the OS, so run a VACUUM FULL after if you actually want the files to shrink.