feat: optimize SQL balance calculation (#1152)

* 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>

crates/cdk-common/src/database/wallet.rs

@@ -96,6 +96,13 @@ pub trait Database: Debug {
         state: Option<Vec<State>>,
         spending_conditions: Option<Vec<SpendingConditions>>,
     ) -> Result<Vec<ProofInfo>, Self::Err>;
+    /// Get balance
+    async fn get_balance(
+        &self,
+        mint_url: Option<MintUrl>,
+        unit: Option<CurrencyUnit>,
+        state: Option<Vec<State>>,
+    ) -> Result<u64, Self::Err>;
     /// Update proofs state in storage
     async fn update_proofs_state(&self, ys: Vec<PublicKey>, state: State) -> Result<(), Self::Err>;