fix(mana-llm): add google-genai to requirements.txt for Docker builds

google-genai was in pyproject.toml but missing from requirements.txt.
The Dockerfile uses pip install -r requirements.txt, so the Google
provider never loaded in production. Now that the key is set and the
cloud tier upgraded to gemini-2.5-flash, the import fires on startup.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

services/mana-llm/requirements.txt

@@ -13,6 +13,9 @@ sse-starlette>=2.2.0
 # Caching (optional)
 redis>=5.2.0
 
+# Google Gemini
+google-genai>=1.0.0
+
 # Metrics
 prometheus-client>=0.21.0