Add provider-aware media usage index foundation

[khoinguyenpham04]

This Roadmap discussion mirrors #1669: Add provider-aware media usage index foundation.

Use this discussion to upvote the roadmap item and discuss priority, use cases, and product feedback. Keep implementation tracking, reproduction details, and PR-specific feedback on the source issue.

• Source issue: #1669
• Status: Closed
• Labels: None
• Original author: khoinguyenpham04

Original Issue

Adds the internal foundation for tracking where media assets are referenced across content.

Scope:

• Add media usage database tables
• Track local media and structured provider asset references
• Index content lifecycle changes: create, update, publish, unpublish, delete, restore, schedule, discard draft, duplicate, revision restore
• Add tests for extraction, repository behavior, migrations, and lifecycle correctness

Out of scope:

• Admin UI
• Public usage API
• Safe-delete behavior
• Replace-in-place UX
• Backfill for existing sites

Acceptance criteria:

• New migrations pass
• Media references are indexed for supported field types
• Usage rows stay correct across content lifecycle operations
• Existing users see no visible behavior change
• Focused tests, typecheck, and lint pass