[feature]: Add OAuth app registration to CE for MCP server support

Is there an existing issue for this?

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Summary

Add OAuth app registration (/auth/o/* endpoints) to Plane CE so that plane-mcp-server's HTTP transport with per-user OAuth works against self-hosted instances. Currently these endpoints only exist on Plane Cloud, leaving self-hosted users with no multi-user auth option for MCP.

Why should this be worked on?

Self-hosted teams have no way to use plane-mcp-server with per-user auth. The MCP server's OAuth flow is already built — it proxies through /auth/o/authorize-app/, /auth/o/token/, and /auth/o/app-installation/ — but these endpoints don't exist in CE. There's no OAuth app model in plane/db/models/, no registration UI, and no way to get a client_id/client_secret. Pointing PLANE_BASE_URL at a self-hosted instance just 404s.

This blocks non-coding use cases — project managers and team leads using MCP-compatible tools like Claude Desktop to interact with Plane, without setting up stdio or sharing a single API token. Ideally the MCP server would be another service in the default docker-compose.yml, with per-user OAuth working the same way it does on Cloud.