feat: add custom OpenAI-compatible provider with custom headers and parameters

- Add Custom provider for any OpenAI-compatible API
- All providers now support custom headers and parameters
- providerSettings stored per-provider with headers and parameters
- Example: disable thinking in Ollama, add tools, etc.

backend/src/services/ai/provider.ts

@@ -6,16 +6,33 @@ export interface AIProviderResult {
 }
 
 export interface AIProvider {
-  provider: 'openai' | 'anthropic' | 'ollama' | 'lmstudio' | 'groq';
+  provider: string;
   generate(prompt: string, systemPrompt?: string, options?: { jsonMode?: boolean }): Promise<AIProviderResult>;
   validate?(): Promise<boolean>;
 }
 
 export interface AIProviderConfig {
-  provider: 'openai' | 'anthropic' | 'ollama' | 'lmstudio' | 'groq';
-  apiKey: string;
+  provider: string;
+  apiKey?: string;
   model?: string;
   baseUrl?: string;
+  providerSettings?: string;
+}
+
+function parseProviderSettings(settingsJson?: string): {
+  headers?: Record<string, string>;
+  parameters?: Record<string, unknown>;
+} {
+  if (!settingsJson) return {};
+  try {
+    const parsed = JSON.parse(settingsJson);
+    return {
+      headers: parsed.headers,
+      parameters: parsed.parameters,
+    };
+  } catch {
+    return {};
+  }
 }
 
 import { OpenAIProvider } from './openai';
@@ -23,19 +40,24 @@ import { AnthropicProvider } from './anthropic';
 import { OllamaProvider } from './ollama';
 import { LMStudioProvider } from './lmstudio';
 import { GroqProvider } from './groq';
+import { CustomProvider } from './custom';
 
 export function createAIProvider(config: AIProviderConfig): AIProvider {
+  const settings = parseProviderSettings(config.providerSettings);
+  
   switch (config.provider) {
     case 'openai':
-      return new OpenAIProvider(config);
+      return new OpenAIProvider(config, settings);
     case 'anthropic':
-      return new AnthropicProvider(config);
+      return new AnthropicProvider(config, settings);
     case 'ollama':
-      return new OllamaProvider(config);
+      return new OllamaProvider(config, settings);
     case 'lmstudio':
-      return new LMStudioProvider(config);
+      return new LMStudioProvider(config, settings);
     case 'groq':
-      return new GroqProvider(config);
+      return new GroqProvider(config, settings);
+    case 'custom':
+      return new CustomProvider(config, settings);
     default:
       throw new Error(`Unknown AI provider: ${config.provider}`);
   }