Add x402 access pass middleware

[DenisCarriere]

Summary

• Add optional x402 V2 middleware for a $0.10 one-hour Token API Pro access pass.
• Accept both EVM and SVM payments via the CDP facilitator, with default pay-to addresses from the tracking issue.
• Configure concrete paid GET routes and explicit free routes, with Bazaar discovery and required payment-identifier extensions.
• Cache settled payment identifiers in memory for one hour and fall back to facilitator verification on cache miss/restart.
• Prevent public cache headers on x402 payment-bearing responses.

Defaults

• Disabled unless X402_ENABLED=true.
• Facilitator: https://api.cdp.coinbase.com/platform/v2/x402
• EVM: eip155:8453, 0x49D581486438aAD93f4114084Ac5B09A8b7C9685
• SVM: solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp, EpRR35QnB5PfTczy3j5rp9bCw4NKzHkd8S1ubdza4my9
• Access plan: pro, price $0.10, duration 3600 seconds

Validation

• bun lint
• bun test --coverage

Refs #494
Closes #495