Sepolia balance and gas routes ignore configured RPC endpoints

The server-side Sepolia routes do not consistently use the RPC endpoint configuration documented for the portal.

Evidence in the repository:

- `.env.example` documents `AGGLAYER_SEPOLIA_RPC_URL` as the primary server RPC endpoint and `EVM_RPC_URL` as the fallback.
- `README.md` documents the same variables for server-side Sepolia RPC usage.
- `src/app/api/sepolia/transaction/route.ts` uses `process.env.AGGLAYER_SEPOLIA_RPC_URL ?? process.env.EVM_RPC_URL ?? "https://ethereum-sepolia-rpc.publicnode.com"`.
- `src/app/api/sepolia/balance/route.ts` and `src/app/api/sepolia/gas/route.ts` hard-code `https://ethereum-sepolia-rpc.publicnode.com` instead.

As a result, deployments that configure a private or rate-limited-safe Sepolia RPC still send balance and gas requests to the public fallback endpoint.

Duplicate check performed before filing:

- `repo:0xMiden/bridge-portal is:open sepolia rpc env balance gas`
- `repo:0xMiden/bridge-portal is:open AGGLAYER_SEPOLIA_RPC_URL balance gas`