When a compliance or security team asks: > “What was the exact state of this invoice on March 3rd at 2:00 PM, and who changed it?” Traditional CRUD systems struggle to answer reliably. Rows get overwritten, timestamps are modified, and historical truth becomes difficult to verify. This poster presents an append-only event sourcing approach for building tamper-evident audit trails in Python. Instead of mutating records in place, every state transition is recorded as an immutable event. Current state becomes a projection of history rather than a mutable database row. The poster walks through: * Why mutable database patterns fail under audit pressure * Designing an append-only event store in PostgreSQL * Reconstructing the historical state through event replay * Detecting tampering using SHA-256 hash chaining * Practical tradeoffs of immutable event architectures * Lightweight Python implementations using Pydantic v2, SQLAlchemy Core, asyncpg, dataclasses, and hashlib The visual design of the poster is structured around a live event stream timeline inspired by Git history and distributed systems diagrams, helping attendees intuitively understand immutability, projections, and integrity verification. The goal is to provide practical engineering guidance for systems where historical integrity matters — particularly fintech, audit, compliance, and security-sensitive applications.