Per-collaborator connector credentials override

[shreydevkar]

Collaborators currently inherit the owner's connector keys (Shopify / Stripe / GitHub). That's right for most cases but sometimes each user's formula should hit their own account. Let each collaborator optionally override the owner's keys with their own.

Acceptance criteria:

• user_plugin_secrets lookup tries the caller first, falls back to the owner if no override.
• Configure modal shows which slots are "yours" vs "inherited from owner" when the workbook is shared.