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>

.env.example

@@ -0,0 +1,11 @@
+# Copy to .env — FLASK_SECRET_KEY is auto-generated on first run if omitted.
+FLASK_SECRET_KEY=
+
+# LLM provider (leave blank to use stub estimation)
+# Supported: gemini
+LLM_PROVIDER=
+
+# Gemini (required when LLM_PROVIDER=gemini)
+# Install: pip install -e '.[gemini]'   (from repo root)
+GEMINI_API_KEY=
+GEMINI_MODEL=gemini-2.0-flash