fix: Improve Supabase signup error handling and auto-confirm emails

CRITICAL PERFORMANCE FIX: Database connection optimization - Increased SQLAlchemy connection pool from 5 to 20 - Added connection recycling (5 min TTL) - Made SupabaseAuth singleton to reuse connections - Changed audit logging from commit() to flush() to batch writes - Added comprehensive performance fix documentation See PERFORMANCE_FIX.md for CRITICAL step: Must update DATABASE_URL to use Supabase connection pooler (port 6543)

feat: implement automatic token refresh for Supabase Free tier

revert: undo ClientOptions implementation and return to stable Supabase init for v2.0.3

fix: replace dict with ClientOptions in Supabase client init to restore stateless config

fix: disable Supabase auto-refresh for stateless backend and switch logging to ISO 8601 timestamps

fix(auth): sync User model and auth responses with actual schema

feat(auth): integrate Supabase Auth and remove local password handling