Document Uploads: New v2 endpoint

[kartpop]

Is your feature request related to a problem?
Document uploads currently take up to ~8 seconds on the backend, which impacts performance. There is a need for a cleaner, more efficient solution for document uploads.

Describe the solution you'd like

• Create a v2 documents endpoint that uses presigned-URL only with clean application/json, no multipart form data.
• Deprecate the v1 documents endpoint.
• Encourage Glific to migrate to v2.

Original issue

Context

Document uploads currently take up to ~8 seconds on the backend. An existing issue covers presigned-URL uploads for the knowledge base; this issue is specifically about creating a clean v2 documents endpoint.

Decision

• Create a v2 documents endpoint that is presigned-URL only — clean application/json, no multipart form data.
• Deprecate the v1 documents endpoint.
• Nudge Glific to migrate to v2.

Rationale

Offloads the backend where uploads currently take up to ~8 seconds.

Acceptance criteria

• v2 endpoint accepts JSON-only, presigned-URL-based upload flow.
• v1 is marked deprecated with a migration path documented for Glific.

Related: #1143