x402: switch default facilitator to openx402.ai + initialise eagerly

Two boot-time gotchas surfaced once we lit the paywall against a
real Base-mainnet recipient:

1. The public x402.org/facilitator only services testnets — its
   /supported manifest does not list eip155:8453. On mainnet boot
   the middleware throws RouteConfigurationError:missing_facilitator.
   Switch the default in config.js to OpenX402's permissionless
   mainnet facilitator (no API key, supports Base USDC with the
   canonical EIP-712 metadata).

2. syncFacilitatorOnStart was wired to false "for boot speed". Without
   the boot-time /supported fetch, the resource server has no
   registered schemes and "Facilitator does not support exact on
   eip155:8453" is thrown at request time — much harder to spot.
   Pass true explicitly and document why above the call.

108/108 tests still green.

Author: Rotwang9000

src/config.js

@@ -72,7 +72,13 @@ export const config = Object.freeze({
 	x402Enabled: asString('X402_ENABLED', '') === '1',
 	x402Network: asString('X402_NETWORK', 'eip155:8453'),
 	x402RecipientAddress: asString('X402_RECIPIENT_ADDRESS', ''),
-	x402FacilitatorUrl: asString('X402_FACILITATOR_URL', 'https://x402.org/facilitator'),
+	// Mainnet facilitator. The public x402.org/facilitator only services
+	// testnets (Base Sepolia / Solana Devnet / etc.) — using it on
+	// eip155:8453 yields "Facilitator does not support exact" at boot.
+	// OpenX402 is permissionless, no API key, supports Base mainnet
+	// USDC with proper EIP-712 metadata. Override at any time via
+	// X402_FACILITATOR_URL.
+	x402FacilitatorUrl: asString('X402_FACILITATOR_URL', 'https://facilitator.openx402.ai'),
 	// Price per call for each premium endpoint, expressed in the x402
 	// Money format ("$0.05" = 5 ¢). The facilitator quotes the USDC
 	// atomic amount on Base from this.

src/x402.js

@@ -142,14 +142,20 @@ export async function registerX402(app, x402Cfg) {
 	]);
 	const facilitatorClient = new HTTPFacilitatorClient({ url: x402Cfg.facilitatorUrl });
 	const schemes = [{ network: x402Cfg.network, server: new ExactEvmScheme() }];
+	// `syncFacilitatorOnStart: true` is required — it fetches the
+	// /supported manifest from the facilitator so the resource server
+	// knows which (scheme, network) tuples it can issue
+	// PaymentRequirements for. Without it the middleware throws on
+	// the first 402-eligible request: "Facilitator does not support
+	// exact on eip155:8453".
 	paymentMiddlewareFromConfig(
 		app,
 		x402Cfg.routes,
 		facilitatorClient,
 		schemes,
 		/* paywallConfig */ undefined,
 		/* paywall */ undefined,
-		/* syncFacilitatorOnStart */ false
+		/* syncFacilitatorOnStart */ true
 	);
 }