feat: implement web UI for provider and model configuration

- Added 'provider_configs' and 'model_configs' tables to database.
- Refactored ProviderManager to support thread-safe dynamic updates and database overrides.
- Implemented 'Models' tab in dashboard to manage model visibility, mapping, and pricing.
- Added provider configuration modal to 'Providers' tab.
- Integrated database overrides into chat completion logic (enabled state, mapping, and cost).

src/providers/gemini.rs

@@ -73,7 +73,10 @@ pub struct GeminiProvider {
 impl GeminiProvider {
     pub fn new(config: &crate::config::GeminiConfig, app_config: &AppConfig) -> Result<Self> {
         let api_key = app_config.get_api_key("gemini")?;
-        
+        Self::new_with_key(config, app_config, api_key)
+    }
+
+    pub fn new_with_key(config: &crate::config::GeminiConfig, app_config: &AppConfig, api_key: String) -> Result<Self> {
         let client = reqwest::Client::builder()
             .timeout(std::time::Duration::from_secs(30))
             .build()?;