Add abstraction for batch jobs

[ahuang11]

I've noticed that we've been embedding a lot more text in recent times (soon Vega-Lite docs), and the previous method of looping could be costly, so here I introduce batch processing for chunk situating. This could also be useful for batch generating multiple reports, or performing evals, in the future

This PR adds multi-provider batch support (OpenAI, Anthropic, Mistral) that processes chunks in parallel rather than sequentially, and has capability to auto-resume the existing batches.

Falls back to sequential processing if batch APIs aren't supported or batch_situate_threshold (default: 1000 chunks, set to 0 to disable).

OpenAI batch docs: https://platform.openai.com/docs/guides/batch
Mistral: https://docs.mistral.ai/capabilities/batch/
Anthropic: https://docs.anthropic.com/en/docs/build-with-claude/batch-processing

[codecov]

Codecov Report

Attention: Patch coverage is 24.44444% with 408 lines in your changes missing coverage. Please review.

Project coverage is 57.18%. Comparing base (9107130) to head (71e9a18).

Files with missing lines	Patch %	Lines
lumen/ai/batch_providers.py	25.25%	219 Missing ⚠️
lumen/ai/batch_situate.py	11.81%	97 Missing ⚠️
lumen/ai/vector_store.py	43.00%	57 Missing ⚠️
lumen/ai/llm.py	10.00%	18 Missing ⚠️
lumen/ai/embeddings.py	0.00%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1290      +/-   ##
==========================================
- Coverage   58.20%   57.18%   -1.02%     
==========================================
  Files         114      116       +2     
  Lines       18318    18818     +500     
==========================================
+ Hits        10662    10762     +100     
- Misses       7656     8056     +400     

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