config: remove hardcoded defaults, add nvidia_enable_thinking setting

All LLM parameters must now be explicitly set in .env — no silent
fallbacks in code. Added nvidia_enable_thinking bool and wired it into
the chat completion extra_body instead of the previous hardcoded True.

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

server/app/config.py

@@ -4,13 +4,14 @@ from pydantic_settings import BaseSettings
 class Settings(BaseSettings):
     nvidia_api_key: str
     app_password: str
-    nvidia_base_url: str = "https://integrate.api.nvidia.com/v1"
-    nvidia_model: str = "nvidia/nemotron-3-ultra-550b-a55b"
-    nvidia_timeout_seconds: int = 120
-    nvidia_max_tokens: int = 16384
-    nvidia_temperature: float = 1.0
-    nvidia_top_p: float = 0.95
-    nvidia_reasoning_budget: int = 2048
+    nvidia_base_url: str
+    nvidia_model: str
+    nvidia_timeout_seconds: int
+    nvidia_max_tokens: int
+    nvidia_temperature: float
+    nvidia_top_p: float
+    nvidia_reasoning_budget: int
+    nvidia_enable_thinking: bool
 
     model_config = {"env_file": ".env"}