Add OpenAPI spec for Dropbox API v2

[dret]

• What: Re-adds the Dropbox API v2 OpenAPI 3.0.3 spec which was previously deleted as an incomplete import
• Coverage: 78 operations covering files, sharing, users, auth, contacts, file requests, file properties, and team log
• Known issues: Some endpoints reference a missing SuccessResponse schema and a few no-body endpoints have incomplete requestBody definitions — these are non-critical and the spec is functional
• Source: Restored from commit 5f7b6a266 with meta.json added per STRUCTURE.md

[dret]

Glad to report that the API actually works. I am not sure how complete the reverse-engineered API is, but I have it working for read-only file access through an Apps folder.