feat(config): migrate configuration management to viper

Replace manual environment variable parsing with Viper to support
file-based configuration.

- Enable loading config from `config.yaml` or `./config/config.yaml`
- Allow custom config path via `EZ_CONFIG_FILE` environment variable
- Bind existing environment variables to maintain backward compatibility
- Update documentation with configuration examples and precedence rules

internal/config/config.go

@@ -1,9 +1,12 @@
 package config
 
 import (
+	"fmt"
 	"os"
-	"strconv"
+	"strings"
 	"time"
+
+	"github.com/spf13/viper"
 )
 
 type Config struct {
@@ -39,50 +42,67 @@ type LogConfig struct {
 }
 
 func Load() (*Config, error) {
-	return &Config{
+	v := viper.New()
+
+	v.SetDefault("server.port", "8080")
+	v.SetDefault("postgres.dsn", "host=localhost user=postgres password=postgres dbname=ezapi port=5432 sslmode=disable")
+	v.SetDefault("redis.addr", "localhost:6379")
+	v.SetDefault("redis.password", "")
+	v.SetDefault("redis.db", 0)
+	v.SetDefault("log.batch_size", 10)
+	v.SetDefault("log.flush_ms", 1000)
+	v.SetDefault("log.queue_capacity", 10000)
+	v.SetDefault("auth.jwt_secret", "change_me_in_production")
+
+	v.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
+	v.AutomaticEnv()
+
+	_ = v.BindEnv("server.port", "EZ_API_PORT")
+	_ = v.BindEnv("postgres.dsn", "EZ_PG_DSN")
+	_ = v.BindEnv("redis.addr", "EZ_REDIS_ADDR")
+	_ = v.BindEnv("redis.password", "EZ_REDIS_PASSWORD")
+	_ = v.BindEnv("redis.db", "EZ_REDIS_DB")
+	_ = v.BindEnv("log.batch_size", "EZ_LOG_BATCH_SIZE")
+	_ = v.BindEnv("log.flush_ms", "EZ_LOG_FLUSH_MS")
+	_ = v.BindEnv("log.queue_capacity", "EZ_LOG_QUEUE")
+	_ = v.BindEnv("auth.jwt_secret", "EZ_JWT_SECRET")
+
+	if configFile := os.Getenv("EZ_CONFIG_FILE"); configFile != "" {
+		v.SetConfigFile(configFile)
+	} else {
+		v.SetConfigName("config")
+		v.SetConfigType("yaml")
+		v.AddConfigPath(".")
+		v.AddConfigPath("./config")
+	}
+
+	if err := v.ReadInConfig(); err != nil {
+		if _, ok := err.(viper.ConfigFileNotFoundError); !ok {
+			return nil, fmt.Errorf("read config: %w", err)
+		}
+	}
+
+	cfg := &Config{
 		Server: ServerConfig{
-			Port: getEnv("EZ_API_PORT", "8080"),
+			Port: v.GetString("server.port"),
 		},
 		Postgres: PostgresConfig{
-			DSN: getEnv("EZ_PG_DSN", "host=localhost user=postgres password=postgres dbname=ezapi port=5432 sslmode=disable"),
+			DSN: v.GetString("postgres.dsn"),
 		},
 		Redis: RedisConfig{
-			Addr:     getEnv("EZ_REDIS_ADDR", "localhost:6379"),
-			Password: getEnv("EZ_REDIS_PASSWORD", ""),
-			DB:       getEnvInt("EZ_REDIS_DB", 0),
+			Addr:     v.GetString("redis.addr"),
+			Password: v.GetString("redis.password"),
+			DB:       v.GetInt("redis.db"),
 		},
 		Log: LogConfig{
-			BatchSize:     getEnvInt("EZ_LOG_BATCH_SIZE", 10),
-			FlushInterval: getEnvDuration("EZ_LOG_FLUSH_MS", 1000),
-			QueueCapacity: getEnvInt("EZ_LOG_QUEUE", 10000),
+			BatchSize:     v.GetInt("log.batch_size"),
+			FlushInterval: time.Duration(v.GetInt("log.flush_ms")) * time.Millisecond,
+			QueueCapacity: v.GetInt("log.queue_capacity"),
 		},
 		Auth: AuthConfig{
-			JWTSecret: getEnv("EZ_JWT_SECRET", "change_me_in_production"),
+			JWTSecret: v.GetString("auth.jwt_secret"),
 		},
-	}, nil
-}
-
-func getEnv(key, fallback string) string {
-	if value, ok := os.LookupEnv(key); ok {
-		return value
 	}
-	return fallback
-}
 
-func getEnvInt(key string, fallback int) int {
-	if value, ok := os.LookupEnv(key); ok {
-		if i, err := strconv.Atoi(value); err == nil {
-			return i
-		}
-	}
-	return fallback
-}
-
-func getEnvDuration(key string, fallbackMs int) time.Duration {
-	if value, ok := os.LookupEnv(key); ok {
-		if i, err := strconv.Atoi(value); err == nil {
-			return time.Duration(i) * time.Millisecond
-		}
-	}
-	return time.Duration(fallbackMs) * time.Millisecond
+	return cfg, nil
 }