Issue Slack tokens

Issues per-installation Slack LightAuth tokens for a project. Use Authorization: Basic base64(projectId:projectSecret). Returns auth (Slack team_id -> LightAuth token) and teams (Slack team_id -> ). All returned tokens share the same TTL (seconds) in expiresIn. The map is the unified-API contract for ‘every workspace this deployment can act on’ — the SDK uses it to fan outbound calls and route inbound events without dev code touching team_id directly. Returns 403 if the project does not have the Slack platform enabled. Returns empty auth/teams maps when there are no active installations.