[Backend] Add Supabase migrations and seed data for local development

[Gbangbolaoluwagbemiga]

Problem

New contributors have no idea what tables need to exist. No migration files are checked in.

What needs to be done

1. Export migrations to `supabase/migrations/`:

   • `0001_create_messages.sql`
   • `0002_create_conversations.sql`
   • `0003_create_notifications.sql`

2. Seed file at `supabase/seed.sql` with sample conversations and notifications

3. Local setup via Supabase CLI:
   ```bash
   supabase start
   supabase db reset # applies migrations + seed
   ```

4. Update CONTRIBUTING.md with local Supabase setup steps

Acceptance criteria

• `supabase db reset` works from clean clone
• All tables exist after reset
• CONTRIBUTING.md updated

Good first issue: SQL only, no blockchain knowledge needed.