Add core configuration and base models

Implement global application configuration and custom Pydantic models
following FastAPI best practices.

Core Configuration (src/core/config.py):
- BaseSettings with environment variable support
- Split configuration by domain (following best practices)
- Ollama connection settings (host, model, timeouts)
- API configuration (host, port, prefix)
- CORS settings
- 20-second streaming timeout per turn
- Cached configuration with @lru_cache

Custom Base Models (src/core/models.py):
- CustomBaseModel for consistent serialization
- ISO datetime formatting
- Alias population support
- Enum value serialization
- Validation on assignment
- serializable_dict() for logging/debugging

Exception Handling (src/core/exceptions.py):
- Base AppException with status codes
- OllamaConnectionError (503)
- OllamaTimeoutError (504)
- ModelNotFoundError (404)
- ValidationError (422)
- OpenAI-compatible error structure

Benefits:
- Consistent configuration across domains
- Type-safe settings with validation
- Easy environment override via .env
- Predictable error responses
- Better debugging with serializable models

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/core/exceptions.py

@@ -0,0 +1,52 @@
+"""
+Global exception definitions.
+Domain-specific exceptions should be in their respective modules.
+"""
+from typing import Any
+
+
+class AppException(Exception):
+    """Base exception for all application errors."""
+    
+    def __init__(
+        self,
+        message: str = "An error occurred",
+        status_code: int = 500,
+        details: dict[str, Any] | None = None,
+    ):
+        self.message = message
+        self.status_code = status_code
+        self.details = details or {}
+        super().__init__(self.message)
+
+
+class OllamaConnectionError(AppException):
+    """Raised when cannot connect to Ollama service."""
+    
+    def __init__(self, message: str = "Cannot connect to Ollama service"):
+        super().__init__(message=message, status_code=503)
+
+
+class OllamaTimeoutError(AppException):
+    """Raised when Ollama request times out."""
+    
+    def __init__(self, message: str = "Ollama request timed out"):
+        super().__init__(message=message, status_code=504)
+
+
+class ModelNotFoundError(AppException):
+    """Raised when requested model is not available."""
+    
+    def __init__(self, model_name: str):
+        super().__init__(
+            message=f"Model '{model_name}' not found",
+            status_code=404,
+            details={"model": model_name}
+        )
+
+
+class ValidationError(AppException):
+    """Raised for validation errors."""
+    
+    def __init__(self, message: str, details: dict[str, Any] | None = None):
+        super().__init__(message=message, status_code=422, details=details)