feat: Implementiere umfassende RESTful API für LLM Gateway

Fügt 21 neue API-Endpoints in 4 Phasen hinzu:

Phase 1 - Foundation (Provider & Models):
- GET /api/providers - Liste aller Provider
- GET /api/providers/{provider} - Provider-Details
- GET /api/models - Liste aller Models mit Filtering/Sorting
- GET /api/models/{provider}/{model} - Model-Details

Phase 2 - Core Features (Credentials, Budget, Pricing):
- GET/POST/PUT/DELETE /api/credentials - Credential-Management
- POST /api/credentials/{id}/test - Connection Testing
- GET /api/budget - Budget-Status mit Projektionen
- GET /api/budget/history - Budget-Historie
- GET /api/pricing - Model-Pricing-Listen
- GET /api/pricing/calculator - Kosten-Kalkulator
- GET /api/pricing/compare - Preis-Vergleich

Phase 3 - Analytics (Usage Statistics):
- GET /api/usage/summary - Umfassende Statistiken
- GET /api/usage/requests - Request-History mit Pagination
- GET /api/usage/requests/{id} - Request-Details
- GET /api/usage/charts - Chart-Daten (4 Typen)

Phase 4 - Account (Account Info & Activity):
- GET /api/account - User-Informationen
- GET /api/account/activity - Activity-Log

Features:
- Vollständige Scramble/Swagger-Dokumentation
- Consistent Error-Handling
- API-Key Authentication
- Filtering, Sorting, Pagination
- Budget-Tracking mit Alerts
- Provider-Breakdown
- Performance-Metriken
- Chart-Ready-Data

Controller erstellt:
- ProviderController
- ModelController
- CredentialController
- BudgetController
- PricingController
- UsageController
- AccountController

Dokumentation:
- API_KONZEPT.md - Vollständiges API-Konzept
- API_IMPLEMENTATION_STATUS.txt - Implementation-Tracking
- API_IMPLEMENTATION_SUMMARY.md - Zusammenfassung und Workflows

laravel-app/app/Http/Controllers/Api/ChatCompletionController.php

@@ -18,13 +18,103 @@ class ChatCompletionController extends Controller
     /**
      * Create a chat completion
      * 
-     * Accepts OpenAI-compatible chat completion requests and routes them to the appropriate 
-     * LLM provider (OpenAI, Anthropic, DeepSeek, Google Gemini, or Mistral AI).
+     * Send messages to an LLM provider and receive completions. This endpoint accepts OpenAI-compatible 
+     * requests and routes them to the appropriate LLM provider (OpenAI, Anthropic, DeepSeek, Google Gemini, 
+     * or Mistral AI).
      * 
-     * The request uses the authenticated user's API keys for the specified provider.
-     * Cost tracking, budget checking, and rate limiting are applied automatically.
+     * The request uses the authenticated gateway user's provider credentials (API keys). Cost tracking, 
+     * budget checking, and rate limiting are applied automatically based on the gateway user's configuration.
      * 
-     * Returns an OpenAI-compatible response with usage statistics and cost information.
+     * ## Authentication
+     * 
+     * Requires a valid API key in the Authorization header:
+     * ```
+     * Authorization: Bearer llmg_your_api_key_here
+     * ```
+     * 
+     * ## Supported Providers
+     * 
+     * - `openai` - OpenAI models (GPT-4, GPT-3.5-turbo, etc.)
+     * - `anthropic` - Anthropic Claude models (Claude 4, Claude 3.5 Sonnet, etc.)
+     * - `google` - Google Gemini models (Gemini Pro, Gemini Flash, etc.)
+     * - `deepseek` - DeepSeek models (DeepSeek Chat, DeepSeek Coder)
+     * - `mistral` - Mistral AI models (Mistral Large, Mistral Medium, etc.)
+     * 
+     * ## Example Request
+     * 
+     * ```json
+     * {
+     *   "provider": "openai",
+     *   "model": "gpt-4o-mini",
+     *   "messages": [
+     *     {
+     *       "role": "system",
+     *       "content": "You are a helpful assistant."
+     *     },
+     *     {
+     *       "role": "user",
+     *       "content": "Hello, how are you?"
+     *     }
+     *   ],
+     *   "temperature": 0.7,
+     *   "max_tokens": 150
+     * }
+     * ```
+     * 
+     * ## Example Response
+     * 
+     * ```json
+     * {
+     *   "success": true,
+     *   "request_id": "req_abc123xyz",
+     *   "provider": "openai",
+     *   "model": "gpt-4o-mini",
+     *   "content": "Hello! I'm doing well, thank you for asking. How can I help you today?",
+     *   "role": "assistant",
+     *   "finish_reason": "stop",
+     *   "usage": {
+     *     "prompt_tokens": 23,
+     *     "completion_tokens": 18,
+     *     "total_tokens": 41
+     *   },
+     *   "cost": {
+     *     "prompt_cost": 0.000023,
+     *     "completion_cost": 0.000054,
+     *     "total_cost": 0.000077
+     *   },
+     *   "response_time_ms": 1245
+     * }
+     * ```
+     * 
+     * ## Error Responses
+     * 
+     * ### Budget Exceeded (402)
+     * ```json
+     * {
+     *   "success": false,
+     *   "error": "budget_exceeded",
+     *   "message": "Monthly budget limit exceeded"
+     * }
+     * ```
+     * 
+     * ### Rate Limit Exceeded (429)
+     * ```json
+     * {
+     *   "success": false,
+     *   "error": "rate_limit_exceeded",
+     *   "message": "Rate limit exceeded. Please try again later.",
+     *   "retry_after": 3600
+     * }
+     * ```
+     * 
+     * ### Provider Error (400-500)
+     * ```json
+     * {
+     *   "success": false,
+     *   "error": "provider_error",
+     *   "message": "Invalid API key for provider"
+     * }
+     * ```
      * 
      * @tags Chat
      * 
@@ -65,7 +155,7 @@ class ChatCompletionController extends Controller
             
         } catch (ProviderException $e) {
             Log::error('Provider error in chat completion', [
-                'user_id' => $request->user()->id,
+                'user_id' => $request->user()->user_id,
                 'provider' => $request->input('provider'),
                 'error' => $e->getMessage(),
             ]);
@@ -78,7 +168,7 @@ class ChatCompletionController extends Controller
             
         } catch (\Exception $e) {
             Log::error('Unexpected error in chat completion', [
-                'user_id' => $request->user()->id,
+                'user_id' => $request->user()->user_id,
                 'error' => $e->getMessage(),
                 'trace' => $e->getTraceAsString(),
             ]);