Feature/external review sharing link

[glimz]

What kind of change does this PR introduce?

Feature + tests + docs update.

This PR introduces a new External Review workflow that allows public approve/reject flows for scheduled posts, with auditability and workflow automation.

Why was this change needed?

Post approval in client/manager workflows currently requires friction (internal access or manual back-and-forth), which slows scheduling and increases operational risk for teams handling external stakeholders.

This change enables secure, token-based public review links so non-Postiz users can approve/reject quickly, while still preserving internal controls (audit trail, replay protection, expiry handling, reminders, and DRAFT fallback on rejection/expiry).

No specific issue link was referenced for this branch; this was implemented as a high-priority workflow enhancement discussed during feature planning/testing.

Other information:

• Discussed implementation strategy iteratively during development/testing sessions (backend, frontend, workflow, and security behavior).
• Feature flags were added for safe rollout:
  • EXTERNAL_REVIEW_ENABLED
  • NEXT_PUBLIC_EXTERNAL_REVIEW_ENABLED
• Follow-up items (separate PR candidates):
  • Browser UI E2E harness (Playwright/Cypress)
  • Metrics dashboard + alerting
  • Confirm/resolve schema drift around OrganizationProviderCredential

Checklist:

Put a "X" in the boxes below to indicate you have followed the checklist;

• I have read the CONTRIBUTING guide.
• I checked that there were not similar issues or PRs already open for this.
• This PR fixes just ONE issue (do not include multiple issues or types of change in the same PR) For example, don't try and fix a UI issue and include new dependencies in the same PR.

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