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)

AGENTS.md

@@ -38,6 +38,14 @@ This file documents the AI agents, tools, and development workflow for the Dance
 - Server management guide
 - API endpoint documentation
 
+### Phase 5: Configuration Management (Completed ✅)
+- Viper integration for configuration
+- Environment variable support with DLC_ prefix
+- Customizable server host/port
+- Configurable shutdown timeout
+- Configuration validation and logging
+- Example configuration file
+
 ## 🛠️ Tools & Technologies
 
 | Component | Technology | Version |
@@ -45,6 +53,7 @@ This file documents the AI agents, tools, and development workflow for the Dance
 | Language | Go | 1.26.1 |
 | Router | Chi | v5.2.5 |
 | Logging | Zerolog | v1.35.0 |
+| Configuration | Viper | v1.21.0 |
 | Testing | Standard Library | - |
 | Dependency Management | Go Modules | - |
 
@@ -58,6 +67,8 @@ DanceLessonsCoach/
 │   └── server/         # Web server
 │       └── main.go
 ├── pkg/
+│   ├── config/         # Configuration management
+│   │   └── config.go    # Viper-based config
 │   ├── greet/          # Core domain logic
 │   │   ├── api_v1.go    # API handlers
 │   │   ├── greet.go     # Service implementation
@@ -66,6 +77,7 @@ DanceLessonsCoach/
 │       └── server.go
 ├── go.mod              # Dependencies
 ├── go.sum              # Dependency checksums
+├── config.example.yaml # Configuration template
 ├── README.md           # User documentation
 ├── AGENTS.md           # This file
 └── .gitignore          # Ignore patterns
@@ -98,6 +110,64 @@ Server running on :8080
 - 30-second shutdown timeout
 - Proper resource cleanup
 
+### Configuration Management
+
+The server supports flexible configuration through environment variables with the `DLC_` prefix:
+
+**Available Configuration Options:**
+
+| Option | Environment Variable | Default Value | Description |
+|--------|---------------------|---------------|-------------|
+| Host | `DLC_SERVER_HOST` | `0.0.0.0` | Server bind address |
+| Port | `DLC_SERVER_PORT` | `8080` | Server listening port |
+| Shutdown Timeout | `DLC_SHUTDOWN_TIMEOUT` | `30s` | Graceful shutdown timeout |
+
+**Usage Examples:**
+
+```bash
+# Custom port
+export DLC_SERVER_PORT=9090
+go run cmd/server/main.go &
+
+# Custom host and port
+export DLC_SERVER_HOST="127.0.0.1"
+export DLC_SERVER_PORT=8081
+go run cmd/server/main.go &
+
+# Custom shutdown timeout
+export DLC_SHUTDOWN_TIMEOUT=45s
+go run cmd/server/main.go &
+```
+
+**Configuration File Support:**
+
+A `config.example.yaml` file is provided as a template:
+
+```yaml
+server:
+  host: "0.0.0.0"
+  port: 8080
+
+shutdown:
+  timeout: 30s
+```
+
+**Configuration Loading:**
+- Environment variables take precedence over defaults
+- All configuration is validated on startup
+- Invalid configurations cause server startup failure
+- Configuration values are logged at startup
+
+**Verification:**
+```bash
+# Test with custom configuration
+DLC_SERVER_PORT=9090 DLC_SERVER_HOST="127.0.0.1" go run cmd/server/main.go
+
+# Verify it's running on the custom port
+curl http://127.0.0.1:9090/api/health
+# Expected: {"status":"healthy"}
+```
+
 ### Checking Server Status
 
 ```bash