feat(media): add flat folder API foundation

[khoinguyenpham04]

What does this PR do?

Adds the backend and typed-client foundation for flat, shared Media Library folders. Local media can belong to one folder or the Main library, list requests can select All media, Main library media, or one folder, and deleting a folder returns its media to the Main library without changing media IDs, storage keys, URLs, or usage records.

This is PR1 of the folders sequence and intentionally has no admin UI, upload-to-folder behavior, nesting, bulk moves, provider changes, CLI commands, or MCP commands. PR2 will add the focused Media Library UI on top of this branch.

This draft is stacked on #2582 so reviewers see only the folder foundation. It will ultimately merge into main after the pagination dependency lands.

Related media roadmap Discussions:

• Make media handling a first-class editing workflow #990
• Add media usage index foundation and my plan for Media usage in EmDash #1655

A folder-specific maintainer-approved Discussion was not found. This PR remains a draft pending scope confirmation.

Type of change

• Bug fix
• Feature (requires maintainer-approved Discussion)
• Refactor (no behavior change)
• Translation
• Documentation
• Performance improvement
• Tests
• Chore (dependencies, CI, tooling)

Checklist

• I have read CONTRIBUTING.md
• Typecheck passes for the changed emdash package
• Type-aware lint passes with 0 diagnostics
• The full default core suite passes: 6,044 tests
• All changed TypeScript and documentation files pass their formatters
• I have added/updated tests for my changes
• User-visible strings in the admin UI are not applicable; PR1 adds no admin UI and includes no messages.po changes
• I have added and reviewed an emdash minor changeset
• New features link to an approved Discussion: folder-specific approval is still needed

AI-generated code disclosure

• This PR includes AI-generated code — model/tool: Codex GPT-5.6, with GPT-5.6 Terra Thinking X High second-opinion reviews

Screenshots / test output

PR1 has no visual changes.

• Full default core suite: 489 files passed, 6,044 tests passed.
• Focused migration, repository, REST, authorization, OpenAPI, and client suite: 223 tests passed.
• Real Astro server client flow passed for folder create/list/rename/delete, assignment, filtering, and safe folder deletion.
• Type-aware lint: 0 diagnostics.
• Core typecheck, core build, OpenAPI validation, changeset validation, and documentation build passed.
• SQLite migration replay and concurrent-migrator coverage passed. PostgreSQL was not configured locally, so dialect-parametric PostgreSQL cases remain for CI.
• Iterative Terra Thinking X High adversarial review converged with no findings.

[changeset-bot]

🦋 Changeset detected

Latest commit: b5b2821

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 17 packages

Name	Type
emdash	Minor
@emdash-cms/cloudflare	Minor
@emdash-cms/sandbox-workerd	Patch
@emdash-cms/plugin-mcp-smoke	Major
@emdash-cms/fixture-perf-site	Patch
@emdash-cms/perf-demo-site	Patch
@emdash-cms/cache-demo-site	Patch
@emdash-cms/do-demo-site	Patch
@emdash-cms/do-solo-demo-site	Patch
@emdash-cms/admin	Minor
@emdash-cms/auth	Minor
@emdash-cms/blocks	Minor
@emdash-cms/gutenberg-to-portable-text	Minor
@emdash-cms/x402	Minor
create-emdash	Minor
@emdash-cms/auth-atproto	Patch
@emdash-cms/plugin-embeds	Patch

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Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions]

Scope check

This PR changes 1,578 lines across 33 files. Large PRs are harder to review and more likely to be closed without review.

If this scope is intentional, no action needed. A maintainer will review it. If not, please consider splitting this into smaller PRs.

See CONTRIBUTING.md for contribution guidelines.

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[github-actions]

Overlapping PRs

This PR modifies files that are also changed by other open PRs:

• fix: activate and automatically index Media Usage #2538 (10 shared files)
• feat(admin): add Media Usage setup and indexing progress #2531 (10 shared files)
• fix(core): add controlled media usage activation #2445 (7 shared files)
• feat(admin): core update notice in the admin dashboard (WordPress-style version banner) #1939 (3 shared files)
• feat(media): add numbered library pagination #2582 (11 shared files)
• feat: reference field type (storage-less edges + admin UI) #1928 (4 shared files)
• feat(core): add getBylines() for listing a site's bylines #2352 (3 shared files)

This may cause merge conflicts or duplicated work. A maintainer will coordinate.