Limit webhook poll batch size

[steve-wecompare]

Summary

• default webhook polling to batches of 5 pending events
• return pending events oldest-first so backlogs drain predictably
• allow an optional ?limit= override, clamped to a bounded range
• add regression coverage for default and explicit poll limits

Tests

• pnpm check passes (0 errors, existing Svelte warnings only)
• pnpm exec vitest run tests/integration/webhook-events.test.ts could not run locally because Testcontainers could not find a working container runtime strategy on this machine