docs: add management API auth Phase 1 design spec and implementation plan

Design specification and implementation plan for Phase 1 of the
management API authentication and authorization system.

Phase 1 covers: pluggable auth framework (AuthN/AuthZ middleware chain),
API key mechanism with SHA-256 hashing, RBAC with fine-grained scopes,
database models (users, roles, API keys), configuration schema, OpenAPI
x-required-scopes annotations, and bootstrapping (config seed + CLI
init-admin command).

Addresses THEEDGE-4735.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Miguel Martín <mmartinv@redhat.com>

Author: mmartinv