feat: make upload attachment folder configurable

[icatw]

Summary

• Adds content.attachmentFolderPath as project config for upload destinations.
• Wires /api/upload through the active project config instead of always using the default page-relative folder.
• Returns markdown-safe src paths relative to the parent document while keeping path as the content-relative stored asset path.
• Adds schema, JSON schema, field registry, and upload behavior coverage.

Verification

• bun test packages/core/src/config/field-registry.test.ts packages/core/src/config/schema-jsonschema.test.ts packages/server/src/config/schema.test.ts packages/server/src/api-extension.test.ts
  • 111 pass / 0 fail
• Real service-level E2E upload with content.attachmentFolderPath: attachments
  • Response included src: "../attachments/screenshot.png"
  • Response included path: "attachments/screenshot.png"
  • New configured asset path existed
  • Old default docs/screenshot.png path did not exist

Verification screenshot:

Full-repo check note

bun run check is currently blocked on the existing public-mirror baseline: oxlint@1.66.0 panics with called Result::unwrap() on an Err value / SIGABRT. I split the unrelated baseline fixes into a separate PR so this feature PR stays focused.

[CLAassistant]

All committers have signed the CLA.