feat(ai): provider router (ollama|gemini|groq) for heavy CV calls

The structured /cv/* calls funnel through a provider router so production can
offload a weak local GPU (GTX 1060) to a cloud provider without any .NET change.
Default stays "ollama" (keyless/local) and /summarize remains local distilbart.

- AI_PROVIDER=ollama|gemini|groq dispatch inside _ollama_generate_json/_text
  (entry-point names kept, so no call sites change; Ollama path is byte-identical).
- Gemini (x-goog-api-key header, not URL query) and Groq (OpenAI-compatible
  chat/completions) added via stdlib urllib — zero new dependencies.
- /health reports ai_provider + ai_provider_configured.
- Keys read from env only; never logged/committed.
- Compose + .env.example pass AI_PROVIDER/GEMINI_*/GROQ_* through.

Tests: 11 passed (default Ollama unchanged, Gemini/Groq dispatch, missing-key 503,
health reports provider).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

@@ -13,6 +13,16 @@ AI_SERVICE_BASE_URL=http://ai-service:8001
 OLLAMA_BASE_URL=http://ollama:11434
 OLLAMA_MODEL=qwen2.5:7b
 
+# AI provider for the heavy /cv/* calls: ollama (default, local) | gemini | groq.
+# /summarize always stays local (distilbart). To offload a weak production GPU,
+# set AI_PROVIDER=gemini (or groq) and provide the matching key below.
+# Keys are read from the environment only — never commit real keys.
+AI_PROVIDER=ollama
+GEMINI_API_KEY=
+GEMINI_MODEL=gemini-2.0-flash
+GROQ_API_KEY=
+GROQ_MODEL=llama-3.3-70b-versatile
+
 # Optional: only needed if you want the UI to call a non-default API base URL.
 # In production the UI defaults to `/api`.
 REACT_APP_API_BASE_URL=