fix(auth): fall back to PLANE_WORKSPACE_SLUG when x-workspace-slug header is absent

Perplexity custom connectors can send only a static API key (Authorization
header) — there is no way to attach x-workspace-slug. With the env fallback
the server resolves the workspace from its own configuration, so
single-workspace deployments work with header-less clients while
multi-workspace clients keep overriding via the header.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>