Add demo application using EventCore with PostgreSQL

[jwilger]

Summary

Create a demonstration application within this repository that showcases EventCore's capabilities with PostgreSQL as the event store backend.

Motivation

A working demo application would:

• Serve as a practical reference for users integrating EventCore
• Demonstrate best practices for event sourcing patterns
• Provide a testbed for validating the library's ergonomics
• Help identify API friction points before they become entrenched

Suggested Scope

• Simple domain (e.g., bank account, todo list, inventory)
• Full event sourcing lifecycle: commands, events, projections
• PostgreSQL event store configuration
• Example read models / projectors
• Docker Compose setup for easy local running

Acceptance Criteria

• Demo application compiles and runs
• README with setup instructions
• Demonstrates at least 2-3 aggregate types
• Shows projection/read model patterns
• Includes integration tests as usage examples