Canonical channel-settings authority and API

[dcellison]

Parent epic: channel settings and shared channel authority.

Problem

The Channel Settings dialog currently fronts standing participation only. Adding unrelated fields directly to that dialog would create multiple ad hoc mutation paths and unclear ownership.

Scope

• Define a canonical channel-settings read model that composes existing channel lifecycle and standing-participation state without creating parallel authorities.
• Add canonical storage only for genuinely new shared fields, with explicit schema versioning and migrations.
• Add authenticated read and mutation APIs with role checks, optimistic concurrency, client_operation_id replay safety, and stable validation errors.
• Publish canonical settings changes through the existing Workshop event stream so multiple open clients converge.
• Define owner, workshop-admin, and participant projections. Unauthorized fields and filesystem details must fail closed.
• Add integrity and replay diagnostics suitable for make install-status.

Acceptance criteria

• A single API projection can populate Channel Settings without client-side joins across unrelated endpoints.
• Existing standing-participation and archive authorities remain authoritative and are referenced rather than copied.
• Concurrent stale writes are rejected deterministically.
• Replaying a successful mutation returns the original result without duplicating events.
• Cross-channel and cross-principal access tests prove isolation.
• Restart/replay tests and installation diagnostics report no gaps.

Non-goals

• Building the final settings dialog.
• Introducing workspace grants or runtime routing before their dedicated issues.