fix(balance): use database credits instead of on-chain balance

Balance checks were reading on-chain Solana token balance (0
during db-only mode). Now wallet and overseer routes return
user.totalEarned - user.totalSpent as the usable credit balance.
This makes credits spendable even when devnet faucet is dry.

- /wallet returns dbBalance, with onChainBalance as extra info
- /overseer/balance returns dbBalance for request eligibility
- /overseer/request checks dbBalance before approving spend
- /wallet/admin returns dbBalance

backend/src/routes/wallet.ts

@@ -65,13 +65,16 @@ router.get(
 			});
 		}
 
-		// Get on-chain balance
-		const balance = await getTokenBalance(user.walletAddress);
+		// Get on-chain balance (may be 0 if using DB-only credits)
+		const onChainBalance = await getTokenBalance(user.walletAddress);
+		// Usable credits are tracked in DB (works even when blockchain is down)
+		const dbBalance = user.totalEarned - user.totalSpent;
 
 		res.json({
 			hasWallet: true,
 			address: user.walletAddress,
-			balance,
+			balance: dbBalance,
+			onChainBalance,
 			totalEarned: user.totalEarned,
 			totalSpent: user.totalSpent,
 			explorerUrl: `https://explorer.solana.com/address/${user.walletAddress}?cluster=devnet`,
@@ -198,14 +201,11 @@ router.get(
 			return res.status(404).json({ error: "User not found" });
 		}
 
-		let balance = 0;
-		if (user.walletAddress) {
-			balance = await getTokenBalance(user.walletAddress);
-		}
+		const dbBalance = user.totalEarned - user.totalSpent;
 
 		res.json({
 			...user,
-			balance,
+			balance: dbBalance,
 		});
 	}),
 );