saas-dr: UpdateSaasIncident RPC for assignee, owner team, summary, severity

[dcoln25-writer]

Phase 2 follow-up.

Once an incident is open, the only mutating RPC is UpdateSaasIncidentStatus. Operators cannot change the assignee, owner team, summary, or severity without recreating the row.

Scope

• Add UpdateSaasIncident(incident_id, assignee_user_id?, owner_team?, summary?, severity?) RPC.
• All fields optional with proto3 optional semantics so the server can distinguish 'not set' from 'set to empty'.
• Re-validate assignee belongs to the caller's organization (same pattern as createSaasIncident).
• Emit a timeline event per change and an audit log row.
• Bump updated_at and last_activity_at.
• Frontend: an 'Edit incident' dialog on incident-detail-page.tsx.

Acceptance

• Tests cover tenant isolation, cross-tenant assignee rejection, partial updates, and that no-op calls do not emit timeline noise.
• Audit log captures which fields changed (old / new).