Add pluggable LLM support with Gemini provider

- Add LLMProvider registry (llm/registry.py) that builds a provider from
  env vars (LLM_PROVIDER, GEMINI_API_KEY, GEMINI_MODEL)
- Add GeminiLLMProvider using the google-genai SDK
- Wire build_llm_provider() into CLI and web pipeline route (replacing llm=None)
- Wrap pass 2 and pass 4 LLM calls in per-combo try/except so API errors
  skip individual combos rather than aborting the whole run
- Add gemini optional dep to pyproject.toml; Dockerfile installs [web,gemini]
- Document env vars in .env.example and README
- Lower requires-python to >=3.10 to match installed system Python

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

Dockerfile

@@ -5,7 +5,7 @@ WORKDIR /app
 COPY pyproject.toml .
 COPY src/ src/
 
-RUN pip install --no-cache-dir ".[web]"
+RUN pip install --no-cache-dir ".[web,gemini]"
 
 VOLUME /app/data
 ENV PHYSCOM_DB=/app/data/physcom.db