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/database/mod.rs

@@ -87,6 +87,8 @@ async fn run_migrations(pool: &DbPool) -> Result<()> {
             enabled BOOLEAN DEFAULT TRUE,
             base_url TEXT,
             api_key TEXT,
+            credit_balance REAL DEFAULT 0.0,
+            low_credit_threshold REAL DEFAULT 5.0,
             updated_at DATETIME DEFAULT CURRENT_TIMESTAMP
         )
         "#