* feat: optimize SQL balance calculation
replace proof-fetching approach with SUM aggregation
- add get_balance() method to Database trait
- implement SQL SUM aggregation in cdk-sql-common
- update total_balance() to use get_balance() instead of get_unspent_proofs()
- redb impl maintains existing behavior
---------
Co-authored-by: thesimplekid <tsk@thesimplekid.com>
Co-authored-by: Cesar Rodas <cesar@rodasm.com.py>