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>

src/physcom/llm/registry.py

@@ -0,0 +1,30 @@
+"""Build an LLMProvider from environment variables."""
+
+from __future__ import annotations
+
+import os
+
+from physcom.llm.base import LLMProvider
+
+
+def build_llm_provider() -> LLMProvider | None:
+    """Return an LLMProvider based on env vars, or None if not configured.
+
+    LLM_PROVIDER   — provider name ('gemini'; more can be added)
+    GEMINI_API_KEY — required when LLM_PROVIDER=gemini
+    GEMINI_MODEL   — optional Gemini model name (default: gemini-2.0-flash)
+    """
+    provider = os.environ.get("LLM_PROVIDER", "").lower().strip()
+
+    if not provider:
+        return None
+
+    if provider == "gemini":
+        api_key = os.environ.get("GEMINI_API_KEY", "")
+        if not api_key:
+            raise ValueError("LLM_PROVIDER=gemini requires GEMINI_API_KEY to be set")
+        model = os.environ.get("GEMINI_MODEL", "gemini-2.0-flash")
+        from physcom.llm.providers.gemini import GeminiLLMProvider
+        return GeminiLLMProvider(api_key=api_key, model=model)
+
+    raise ValueError(f"Unknown LLM_PROVIDER: {provider!r}. Supported: gemini")