Tig 256 seed demo flows

[rohanmatta11]

• Reseeding now refreshes existing events' dates instead of skipping them,
  so events don't silently go stale/past on a long-lived local DB.
• Added the missing "Events RSVP'd" tab to My Events.
• Fixed db:seed throwing DATABASE_URL not set on a fresh checkout (ran
  via bun instead of tsx, which fixed .env inheritance).
• Added root-level bun run db:seed and updated README setup docs.

[linear]

TIG-256

[angelina-ji]

Potential issue with matching seeded events by title

Right now, reseeding looks up existing events using only the event title:
const existingId = existingEventsByTitle.get(e.title);

Since titles aren't guaranteed to be unique, this could update the wrong event if a user-created event (or another seeded event) has the same title. The Map would also only keep one of multiple events with the same title.
Could we instead identify seeded events using something deterministic (e.g. a stable seed ID, slug, or isSeedData flag), or match on a more specific key? That would make reseeding much safer and avoid accidentally modifying real user data.

[angelina-ji]

Could you add the QA results for the other MVP flows to the PR description? In addition to Explore, the task asks us to verify ranking/interests, Event Detail metadata, My Events saved and RSVP’d states, tags, locations, and organizations/sources. A brief checklist noting which seeded user was tested and what was confirmed would make it easier to verify the definition of done.