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[NAH-23] handle bad inputs: empty / image-only / corrupt PDF -> clear error (#11)

[NAH-22] regenerate(card, harder/easier) helper for the difficulty dial (#10)

[NAH-21] tighten generation prompt: ground questions in the passage (#9)

[NAH-20] de-dupe near-identical questions + drop low-quality cards (#8)

[NAH-18] generate_deck: add repair-retry to per-chunk JSON parsing (#7)

[NAH-16] Commit content_pipeline.py stubs

[NZH-17] extract_text (PDF + pasted/.txt) + length-based chunker

Add app, content pipeline, learning engine, and gitignore