Refactor data access to enforce guild-scoped multi-tenancy

[bridgetthnkechi87-cloud]

Closes #298

Summary

• Enforced guild-scoped activity repository access by requiring guildId for append/query operations
• Updated mock and durable repository adapters so activity events are stored and queried per guild
• Wired API/activity writers to pass the correct guild context
• Added a dashboard migration for activity_events.guild_id
• Added repository contract tests proving activity queries cannot leak across guilds
• Documented the multi-tenancy model and known follow-up gaps in docs/multi-tenancy.md

Testing

• pnpm --filter @guildpass/dashboard test apps/dashboard/test/repositories/mock-contract.test.ts apps/dashboard/test/repositories/durable-contract.test.ts apps/dashboard/test/dashboard-activity.test.ts
• Result: 69 passed, 0 failed

Notes

This PR keeps the scope focused on the repository/query choke point for members, passes, and activity. Webhook-ingested activity and SSE live-stream scoping are documented as follow-up gaps rather than expanded into this PR.