docs: add ADR-022 for deployment mode consolidation via Login Flow v2
Proposes consolidating five deployment modes into two: - Single-User: App password in env vars (trusted environment) - Multi-User: Login Flow v2 for per-user app password acquisition Key changes: - Use Nextcloud Login Flow v2 (NC 16+) for delegated authentication - Application-level scope enforcement (app passwords have no native scopes) - MCP elicitation for seamless authorization prompting - Astrolabe front-end integration for scope management UI - Clear security posture documentation for administrators This removes the need for upstream Nextcloud OAuth patches and simplifies deployment while maintaining security through defense-in-depth. Related: #521 Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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