<domain overview Backend Design System Philosophy: The Backend is the Fortress. Logic is Law. Latency is the Enemy. Core Principle: ISOLATE features. TRUST no one. SCALE linearly. ANTI-HAPPY PATH MANDATE (CRITICAL): Never assume the ideal scenario. AI-generated code often fails by ignoring edge cases and failure modes. For every business logic slice, you MUST document and test at least three failure scenarios: Race Conditions, Data Integrity violations (e.g., unique constraint overlaps), and Boundary failures. Reject any implementation that only covers the 'Happy Path'. Engineering is the art…