fix(admin): LLM provider/model switching now reads DB overrides correctly

- get_llm_provider() now accepts provider + model args so DB overrides
  propagate through to all provider constructors (was always reading
  env vars, ignoring the admin UI selection)
- /test-llm replaced with lightweight ping (max_tokens=20) instead of
  running a full bill analysis; shows model name + reply, no truncation
- /api/settings/llm-models endpoint fetches available models live from
  each provider's API (OpenAI, Anthropic REST, Gemini, Ollama)
- Admin UI model picker dynamically populated from provider API;
  falls back to manual text input on error; Custom model name option kept
- Default Gemini model updated: gemini-1.5-pro → gemini-2.0-flash

Co-Authored-By: Jack Levy

backend/app/config.py

@@ -34,7 +34,7 @@ class Settings(BaseSettings):
     ANTHROPIC_MODEL: str = "claude-opus-4-6"
 
     GEMINI_API_KEY: str = ""
-    GEMINI_MODEL: str = "gemini-1.5-pro"
+    GEMINI_MODEL: str = "gemini-2.0-flash"
 
     OLLAMA_BASE_URL: str = "http://host.docker.internal:11434"
     OLLAMA_MODEL: str = "llama3.1"