0-to-1 SaaS Engineering

Learn the core multi-tenant SaaS engineering patterns by fixing real, working bugs: Postgres row-level security as a database-enforced tenant boundary, JWT authentication vs. tenant-scoped authorization, idempotent billing webhook handling with proration math, per-tenant rate limiting, and fair multi-tenant background job queues -- every vulnerable/fixed pair demonstrated with live before/after results, not just description.

Builds on: PostgreSQL 16 + Redis 7.0 + PyJWT 2.7 + psycopg2