feat(ai): define a local inference adapter and deterministic test backend #81

Separate OffPDF product code from the selected inference implementation.

Scope

• Define a small interface for load, unload, generate, cancel, status, and health checks.
• Map runtime failures into normal structured app errors.
• Provide a deterministic fake backend for frontend and integration tests.
• Keep model acquisition and document processing outside this layer.

Acceptance criteria

• Product features do not import a concrete runtime directly.
• Cancellation and failure paths are testable without a real model.
• The adapter exposes no network or arbitrary-command capability.