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/models.py

@@ -0,0 +1,43 @@
+"""
+Custom Pydantic base models for consistent serialization.
+Following best practice of having a global base model.
+"""
+from datetime import datetime
+from typing import Any
+
+from fastapi.encoders import jsonable_encoder
+from pydantic import BaseModel, ConfigDict
+
+
+def datetime_to_iso_str(dt: datetime) -> str:
+    """Convert datetime to ISO format string."""
+    return dt.isoformat()
+
+
+class CustomBaseModel(BaseModel):
+    """
+    Custom base model with consistent configuration.
+    
+    All domain models should inherit from this for:
+    - Consistent JSON serialization
+    - Timezone-aware datetime handling
+    - Alias population support
+    """
+    model_config = ConfigDict(
+        json_encoders={datetime: datetime_to_iso_str},
+        populate_by_name=True,
+        use_enum_values=True,
+        validate_assignment=True,
+        arbitrary_types_allowed=True,
+    )
+    
+    def serializable_dict(self, **kwargs: Any) -> dict[str, Any]:
+        """
+        Return dict with only JSON-serializable fields.
+        
+        Useful for logging and debugging.
+        """
+        return jsonable_encoder(
+            self.model_dump(**kwargs),
+            custom_encoder={datetime: datetime_to_iso_str}
+        )