feat: add ts-to-zod schema generation from spec.types.ts

Add automated Zod schema generation using ts-to-zod:
- New npm script `generate:schemas` to regenerate schemas from spec.types.ts
- Post-processing script for Zod v4 compatibility (zod/v4 import, looseObject)
- src/schemas.generated.ts: Auto-generated schemas
- src/schemas.ts: Re-exports with PascalCase naming for backwards compatibility

The generated schemas provide simpler validation without the custom refinements
in types.ts (like Base64 validation), making them useful for lighter validation
scenarios or as reference implementations.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ochafik