Refactor codebase to support multimodal inputs (image, video, audio)

[nv78]

Schema:

• documents: add media_type ENUM and mime_type, make document_text nullable
• chat_share_documents: same nullable/media_type changes
• Add message_attachments table for per-message media files

Backend:

• db.py: add_document now accepts media_type/mime_type; add
  add_message_attachment and get_message_attachments helpers
• documents/handler.py: detect MIME type of each uploaded file and
  route images/video/audio to binary-safe path (skipping Tika text
  extraction); text documents use the existing Tika pipeline
• agents/config.py: add OPENAI/ANTHROPIC vision model names,
  ENABLE_MULTIMODAL flag, and size limits for each media type
• agents/reactive_agent.py: _initialize_llm accepts a vision flag;
  process_query_stream accepts optional media_attachments list and
  switches to the vision-capable model when attachments are present
• app.py: extract _parse_message_request() to handle both JSON and
  multipart/form-data bodies; pass media_attachments through to agent

Frontend:

• FileUpload.js: extend default accepted types to include images, video,
  and audio; add getMediaCategory helper; show image thumbnails via
  object-URL previews; revoke URLs on remove; category badge in file list
• RequestConfig.js: add postFormData() for multipart uploads (omits
  Content-Type so the browser sets the correct boundary)

https://claude.ai/code/session_01C9mHttiQ4ZAaBbQecVV7uu