Sponsor self-service portal

## Context
PRD §3.1 — Sponsors can log in to update their company info, logo, and view sponsorship history.

## Requirements
- Sponsors can log in (OAuth or invite-based)
- Update company name, URL, logo
- Submit sponsorship interest for specific events
- View their sponsorship history across events

## References
- PRD §3.1 — Sponsor Self-Service Portal