fix: SSE ticket exchange + on-chain payment verification

[rz1989s]

Summary

• HIGH: SSE ?token= query param leaks JWT to proxy logs and browser history #101: SSE ticket exchange — short-lived one-time tickets replace JWT in EventSource URLs. Prevents JWT exposure in proxy logs, browser history, and Referer headers.
• LOW: pay/:id/confirm accepts any txSignature without on-chain verification #120: On-chain transaction verification for payment link confirmation. Validates tx exists, succeeded, involves correct stealth address, and transfers correct amount before marking paid.

Changes

• auth.ts: Added POST /auth/sse-ticket endpoint + consumeSseTicket() + ticket support in verifyJwt
• pay.ts: Added verifyTransaction() with RPC lookup + amount/address validation (fail-open on RPC errors)
• sse.ts + useSSE.ts: Frontend exchanges JWT for ticket before opening EventSource
• pay-route.test.ts: 15 new tests for on-chain verification scenarios

Test plan

• 573 tests passing
• Frontend build clean
• SSE stream connects via ticket (not JWT in URL)
• Payment confirm rejects fake tx signatures