fix: add fallback defaults to Settings and fix limiter import in auth router

fix: resolution of critical blockers (P0/P1/P2 fixes) - async Redis, atomic credit updates, schema compatibility, task event loop safety, rate limits, timezone datetimes

Fix database deadlocks, timezone cookie offsets, and add WeasyPrint GTK fallback safeguards

feat: replace Firebase with Supabase JWT auth (ES256/JWKS + HS256 fallback)