[CHORE] Add foreign key to `target_instance.server_id` so that only valid server assignments or null are allowed

Add foreign key to target_instance.server_id so that only valid server assignments or null are allowed. Discovered by accident while testing target instances
```sql
insert into target_instance (player_id,target_id,server_id,ip) 
select id as player_id, (id % 15)+1 as target_id,(id%6)+1 as server_id,null from player
```