API chat endpoints

Upgraded from ancient langchain, added langserve to serve langchain chains (creates several endpoints where you can invoke or stream results, including a test playground).

• FastAPI + langserve API key secured endpoints (/chat, /rag_chat) along "Proposal Insights" 1st step /similarity_search
• moved from ChromaDB to PGVector
• added PostgresChatMessageHistory

TODO where UX discussion is needed:

• extend PostgresChatMessageHistory to have extra columns in the db table: for user id, other UI grouping/namespace, and other metadata (user feedback, TruLens/RAGAS evaluation of response)
• add chat history retrieval endpoints, per user, per collection?/widget?/namespace?
• add chat history delete/soft-delete/archive endpoint?
• add LLM option in chats (now it defaults to OpenAI, which can be instrumented via a LLM Proxy to use a LocalAI endpoint - see LLM-infra)

TODO:

• control context window size (depends on LLMs used/selected by user?) with context compression
• "duplicate" /rag_chat to "Proposal Insights"-like 2nd step document handling (map/reduce or rolling context compression)

OPTIONAL:

• add Langfuse instrumentation (to log queries, prompts, and even later control/swap prompts)