Fix Langfuse dependency incompatibility and upgrade to Python SDK v4

Hi Quivr team — I'm Hassieb from the Langfuse team.

I noticed Quivr declares langfuse>=2.57.0 without an upper bound while core/quivr_core/rag/utils.py imports CallbackHandler from the removed langfuse.callback path. A fresh install can therefore resolve v4 and fail at import time today.

The concrete fix looks like:

• First add a safe upper bound, then migrate to langfuse>=4.7,<5.
• Import CallbackHandler from langfuse.langchain.
• Keep the handler connected to both the synchronous invoke and asynchronous/streaming code paths.
• Carry user/session/trace correlation through the v4-supported LangChain metadata or propagate_attributes() path.
• Add a clean-environment import test plus one sync and one streaming trace test.

The migration guides are v2→v3 and v3→v4.

If you need support with the migration, we're happy to jump on a call or help review a PR.