Dropbox: return video durations and image thumbnails

[mbyrdLCS]

The Dropbox provider returned no seconds and no thumbnail on any file, so consumers (B1Admin Service Order, FreePlay) showed placeholder icons and 0:00 for everything.

What changed

• list_folder is now called with include_media_info: true, and video files map media_info.metadata.duration (ms) → ContentFile.seconds. Guarded for media_info: pending (Dropbox still processing).
• Images use their shared raw link as thumbnail — it's the same URL already being created, so zero additional API calls.
• Images deliberately get no duration and videos no thumbnail: image display length is an operator decision (FreePlay keeps them up until advanced), and Dropbox video thumbnails require an authed byte fetch that can't be expressed as a URL — consumers can paint a first frame from url instead.

Testing

• yarn build clean, all 21 existing tests pass
• Companion UI change consuming this data: ChurchApps/B1Admin PR #415

🤖 Generated with Claude Code

[mbyrdLCS]

Note on the failing CI check: this is pre-existing breakage on main, not from this PR. Every run on main today fails identically — the apihelper workspace can't resolve @churchapps/helpers (TS2307 in src/helpers/index.ts) and the topological build stops there before reaching content-providers. This PR only touches content-providers/src/providers/dropbox; locally the package builds clean and all 21 tests pass.