Provide way to re-embed entire database if the embedding model is changed

[zackify]

if the float embedding size has changed, migrate the sqlite database first, make sure to dump or truncate the existing rows, we dont care about them, we will re-embed after doing this part

add a re-embed endpoint that takes a model prop and embedding size, and uses the generate endpoint to loop over all chunks in the document_chunks table and all the documents table to rembed them with this different model.