feat(config): support cli config path and nested config

Adds -config CLI flag to main.go. Moves ParseLogLevel to internal/config. Adds ServerConfig nested struct to Config and corresponding tests for yaml/env overrides.

internal/config/config.go

@@ -2,6 +2,7 @@ package config
 
 import (
 	"fmt"
+	"log/slog"
 	"os"
 	"reflect"
 	"strconv"
@@ -13,7 +14,14 @@ import (
 
 // Config holds the application configuration.
 type Config struct {
-	LogLevel string `yaml:"log_level"`
+	LogLevel string       `yaml:"log_level"`
+	Server   ServerConfig `yaml:"server"`
+}
+
+// ServerConfig holds server-specific configuration.
+type ServerConfig struct {
+	Host string `yaml:"host"`
+	Port int    `yaml:"port"`
 }
 
 // Load reads configuration from a YAML file and overrides it with environment variables.
@@ -42,6 +50,22 @@ func Load(configPath string, envPrefix string) (*Config, error) {
 	return cfg, nil
 }
 
+// ParseLogLevel converts a string log level to a slog.Level.
+func ParseLogLevel(level string) slog.Level {
+	switch strings.ToLower(level) {
+	case "debug":
+		return slog.LevelDebug
+	case "info":
+		return slog.LevelInfo
+	case "warn":
+		return slog.LevelWarn
+	case "error":
+		return slog.LevelError
+	default:
+		return slog.LevelInfo
+	}
+}
+
 // overrideFromEnv uses reflection to traverse the struct and update fields from env vars.
 func overrideFromEnv(cfg interface{}, prefix string) error {
 	v := reflect.ValueOf(cfg)