refactor: unify storage layer and modularize Gradio web app

feat: ZeroGPU backend for Hugging Face Spaces deployment (#4)

chore: hardening

docs: add ZeroGPU queue architecture ADR and design document

docs: add ZeroGPU ADR and context documentation

docs: overhaul README, update ADR for 6-stage pipeline, supersede feasibility studies

chore: renumber docs and adr for consistent ordering

feat: SmolLM2-135M ranker fine-tuning pipeline + BAML streaming improvements

feat: production-ready DenseFeed pipeline with jj workflow docs