Add typed export-template lifecycle and execution support

[terjekv]

Gap

Exports have a high-level typed service, but all eight export-template operations remain generic:

• list/create/get/update/delete templates;
• run a template-backed export;
• template events, history, and as-of reads.

This leaves reusable exports less safe and less discoverable than one-off ExportRequest workflows.

Acceptance criteria

• Add ExportTemplateId, strict create/update/run request models, and forward-compatible template response models.
• Expose a cursor-aware client.export_templates service with CRUD and execution.
• Return the existing typed Task from template execution and interoperate with client.exports output retrieval.
• Add typed template audit/history methods or integrate with the shared audit/history service.
• Preserve exact sync/async parity.
• Add unit and pinned v0.0.9 lifecycle/execute coverage.
• Document template rendering, missing-data policy, output negotiation, and task workflow.