docs: define fleet control plane PRD roadmap

[matteius]

Summary

• split the fleet-scale audit into eleven independently pursuable PRDs
• establish the dependency order from camera identity through advanced ONVIF and later operational features
• define provider-neutral MQTT event routing instead of native email/SMS integrations
• explicitly defer OIDC/SSO until a committed organizational deployment supplies a real IdP and testing partner

Review guidance

The most consequential decisions are the shared selector model, the separation of storage targets from lifecycle policies, and the boundary between core events and external notification delivery.

Validation

• relative PRD links resolve
• every new PRD includes problem, requirements, phases, acceptance criteria, risks, and dependencies
• git diff --check passes