feat: implement provider credit tracking and balance management

- Added 'credit_balance' and 'low_credit_threshold' to 'provider_configs' table.
- Updated dashboard backend to support reading and updating provider credits.
- Implemented real-time credit deduction from provider balances on successful requests.
- Added visual balance indicators and configuration modal to the 'Providers' dashboard tab.

src/logging/mod.rs

@@ -56,6 +56,8 @@ impl RequestLogger {
 
     /// Insert a log entry into the database
     async fn insert_log(pool: &SqlitePool, log: RequestLog) -> Result<(), sqlx::Error> {
+        let mut tx = pool.begin().await?;
+
         sqlx::query(
             r#"
             INSERT INTO llm_requests 
@@ -65,7 +67,7 @@ impl RequestLogger {
         )
         .bind(log.timestamp)
         .bind(log.client_id)
-        .bind(log.provider)
+        .bind(&log.provider)
         .bind(log.model)
         .bind(log.prompt_tokens as i64)
         .bind(log.completion_tokens as i64)
@@ -77,9 +79,22 @@ impl RequestLogger {
         .bind(log.duration_ms as i64)
         .bind(None::<String>) // request_body - TODO: store serialized request
         .bind(None::<String>) // response_body - TODO: store serialized response or error
-        .execute(pool)
+        .execute(&mut *tx)
         .await?;
 
+        // Deduct from provider balance if successful
+        if log.cost > 0.0 {
+            sqlx::query(
+                "UPDATE provider_configs SET credit_balance = credit_balance - ? WHERE id = ?"
+            )
+            .bind(log.cost)
+            .bind(&log.provider)
+            .execute(&mut *tx)
+            .await?;
+        }
+
+        tx.commit().await?;
+
         Ok(())
     }
 }