Add deployment script for Optimism Sepolia

[iarturo]

Descripción:
ElaraPay currently deploys only to Base Sepolia.
Add a deployment script for Optimism Sepolia.

Tasks:

• Add Optimism Sepolia RPC to foundry.toml
• Add OPTIMISM_SEPOLIA_RPC_URL to .env.example
• Verify the deploy script works on Optimism Sepolia

Acceptance criteria:

• forge script deploys successfully to Optimism Sepolia
• Contract verified on Optimism Sepolia explorer