refactor: centralize outbox event types

[dollarop]

Closes #179

Summary

• Adds shared OUTBOX_EVENT_TYPES, OutboxEventType, and OUTBOX_EVENT_TYPE_VALUES in @guildpass/shared-types.
• Replaces access-api outbox event-type string literals with shared constants in outbox producers/consumers.
• Adds a focused test that verifies the shared event type list is unique and guards the main outbox modules from raw event-type literals.

Validation

• pnpm --filter @guildpass/shared-types build
• pnpm --filter access-api test -- outboxEventTypes.test.ts outboxService.test.ts
• git diff --check

Notes

• Full access-api typecheck currently hits a TypeScript RangeError: Maximum call stack size exceeded in this checkout.
• Full lint has pre-existing errors outside this focused refactor; the changed behavior is covered by the targeted tests above.