chore: switch to DeepSeek API + sentence-transformers for free deployment

- Replace OpenAI LLM with DeepSeek (deepseek-chat) via OpenAI-compatible client
- Replace OpenAI embeddings with local sentence-transformers (all-MiniLM-L6-v2)
- Pre-download embedding model during Docker build
- Add .gitignore

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,7 @@
+.env
+data/*
+!data/.gitkeep
+__pycache__/
+*.pyc
+*.pyo
+.DS_Store