Reworked welcome emails and per-workspace addon management

We have reworked how new users receive their first email from FastPix and how addons are turned on and off for a workspace.

Welcome email pipeline:

• Welcome emails are now scheduled into a per-user email history table at signup time, with the rest of the onboarding nudges chained off the same record.
• The verification, invite-admin, and admin-invoice email templates were cleaned up to remove redundant copy, fix font sizing, and use environment-driven logo URLs so we can re-brand without redeploying.
• A separate signup-notification email is now delivered to FastPix admins so internal teams can react to high-value signups in real time.

Workspace addons API:

• New endpoints to enable, disable, and delete a workspace addon. The delete endpoint requires the addon ID so revocation is precise.
• workspace_addons schema was rewritten to support the new state machine (enabled, disabled, deleted) and reused by DRM, Cloud Playout, and AI Agents.
• A method to retrieve workspace access tokens with optional revoked status was added so dashboards can show both active and historical tokens.

These changes are backwards-compatible at the API level.