Add configuration management with Viper

- Implement pkg/config package with Viper integration
- Support environment variables with DLC_ prefix
- Add configurable server host, port, and shutdown timeout
- Update server to use configuration values
- Add config.example.yaml template file
- Update AGENTS.md with configuration documentation
- Maintain backward compatibility with default values

Configuration options:
- DLC_SERVER_HOST (default: 0.0.0.0)
- DLC_SERVER_PORT (default: 8080)
- DLC_SHUTDOWN_TIMEOUT (default: 30s)

config.example.yaml

@@ -0,0 +1,23 @@
+# DanceLessonsCoach Configuration Example
+# This file shows the available configuration options
+# You can use this as a template for your own configuration
+
+# Server configuration
+server:
+  # Host address to bind to (default: "0.0.0.0")
+  host: "0.0.0.0"
+  
+  # Port to listen on (default: 8080)
+  port: 8080
+
+# Shutdown configuration
+shutdown:
+  # Timeout duration for graceful shutdown (default: 30s)
+  # Format: number + unit (s, m, h)
+  timeout: 30s
+
+# Environment Variables
+# You can also configure via environment variables with DLC_ prefix:
+# DLC_SERVER_HOST=0.0.0.0
+# DLC_SERVER_PORT=8080
+# DLC_SHUTDOWN_TIMEOUT=30s