docs(communities): add blob upload proxy system design

Adds design documentation for blob upload proxy system to enable
image/video posts in communities from external PDS users.

Problem:
Users on external PDSs cannot directly upload blobs to community-owned
PDS repositories because they lack authentication credentials for the
community's PDS.

Solution:
Coves AppView acts as an authenticated proxy for blob uploads via
social.coves.blob.uploadForCommunity endpoint.

Flow:
1. User uploads blob to AppView
2. AppView validates user can post to community
3. AppView uses community's PDS credentials to upload blob
4. AppView returns CID to user
5. User creates post record referencing the CID
6. Post and blob both live in community's PDS

Status: Design documented, implementation TODO
Priority: CRITICAL for Beta - Required for rich media posts

Implementation checklist includes:
- Handler endpoint
- User authorization validation
- Community credential management
- Upload proxy logic
- Security measures (size limits, content-type validation, rate limiting)