setup: add internal package directory structure

[Lewiskunta]

Creates the internal/ package structure per the Tech Stack document.

Packages created:

• internal/api - HTTP handlers, routing, middleware
• internal/auth - Authentication, API key management
• internal/audit - Audit log service
• internal/breach - Breach detection scheduler and logic
• internal/contract - Contract domain logic
• internal/config - Environment variable loading (Viper)
• internal/db - Database layer
• internal/notification - Email and Slack notification service
• internal/proposal - Proposal domain logic
• internal/schema - Schema introspection (Postgres)
• internal/testutil -Shared test helpers

All directories contain a .gitkeep placeholder. Real code is added in Phase 1 as each package is implemented.