Chat attachments

Allow to have file attachments in chat. File attachments must be parsed the same way tender documents do

The goal is to be able to "chat with documents" as if they existed for the tender

~~@xndyz takes care of UI~~

• [X] Create tables for conversation attachments (join table), file contents, file chunks (just a suggestion)
• [X] Same steps as for tender docs: content extraction, summarization, vectorization/chunking
• [X] Create a chat "tool" to search though these documents, summaries etc
• [ ] Create endpoints for FE to upload
• [ ] 25 files per conversation limit
• [ ] Check max char length and round that up to a round number 6785 => 7500

for uploads we take multipart files[] body - json string