feat(db): initialize Supabase project structure and core database migrations #5

Summary

Establish the Supabase project configuration and initial database migrations for core ProblemSpot entities with Row Level Security (RLS) policies as specified in docs/ARCHITECTURE.md (Section 5).

Scope

1. Initialize supabase/ structure:
   • supabase/config.toml
   • supabase/migrations/
   • supabase/seed.sql
2. Create initial migration for core tables:
   • profiles (id, email, username, display_name, avatar_url, github_username, role, is_public)
   • categories (id, name, slug, description, icon)
   • problems (id, title, description, category_id, creator_id, location_name, status, created_at)
   • problem_votes (id, problem_id, user_id, vote_type: personal_confirmation | community_support) with unique constraint on (problem_id, user_id, vote_type)
   • evidence (id, problem_id, user_id, file_url, file_type, description, is_sensitive)
   • problem_status_history (id, problem_id, changed_by, old_status, new_status, reason)
3. Implement Row Level Security (RLS) policies for each table.

Acceptance Criteria

• Migrations apply cleanly.
• Database-level unique constraint prevents duplicate voting.
• RLS policies enforce authenticated writes and public read access where appropriate.