Add initial backend implementation

- Go module with Gin, GORM, JWT, excelize dependencies
- Configuration loading from YAML with all settings
- GORM models for users, categories, components, configurations, alerts
- Repository layer for all entities
- Services: auth (JWT), pricing (median/average/weighted), components,
  quotes, configurations, export (CSV/XLSX), alerts
- Middleware: JWT auth, role-based access, CORS
- HTTP handlers for all API endpoints
- Main server with dependency injection and graceful shutdown

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/config/config.go

@@ -0,0 +1,176 @@
+package config
+
+import (
+	"fmt"
+	"os"
+	"time"
+
+	"gopkg.in/yaml.v3"
+)
+
+type Config struct {
+	Server        ServerConfig        `yaml:"server"`
+	Database      DatabaseConfig      `yaml:"database"`
+	Auth          AuthConfig          `yaml:"auth"`
+	Pricing       PricingConfig       `yaml:"pricing"`
+	Export        ExportConfig        `yaml:"export"`
+	Alerts        AlertsConfig        `yaml:"alerts"`
+	Notifications NotificationsConfig `yaml:"notifications"`
+	Logging       LoggingConfig       `yaml:"logging"`
+}
+
+type ServerConfig struct {
+	Host         string        `yaml:"host"`
+	Port         int           `yaml:"port"`
+	Mode         string        `yaml:"mode"`
+	ReadTimeout  time.Duration `yaml:"read_timeout"`
+	WriteTimeout time.Duration `yaml:"write_timeout"`
+}
+
+type DatabaseConfig struct {
+	Host            string        `yaml:"host"`
+	Port            int           `yaml:"port"`
+	Name            string        `yaml:"name"`
+	User            string        `yaml:"user"`
+	Password        string        `yaml:"password"`
+	MaxOpenConns    int           `yaml:"max_open_conns"`
+	MaxIdleConns    int           `yaml:"max_idle_conns"`
+	ConnMaxLifetime time.Duration `yaml:"conn_max_lifetime"`
+}
+
+func (d *DatabaseConfig) DSN() string {
+	return fmt.Sprintf("%s:%s@tcp(%s:%d)/%s?charset=utf8mb4&parseTime=True&loc=Local",
+		d.User, d.Password, d.Host, d.Port, d.Name)
+}
+
+type AuthConfig struct {
+	JWTSecret     string        `yaml:"jwt_secret"`
+	TokenExpiry   time.Duration `yaml:"token_expiry"`
+	RefreshExpiry time.Duration `yaml:"refresh_expiry"`
+}
+
+type PricingConfig struct {
+	DefaultMethod       string `yaml:"default_method"`
+	DefaultPeriodDays   int    `yaml:"default_period_days"`
+	FreshnessGreenDays  int    `yaml:"freshness_green_days"`
+	FreshnessYellowDays int    `yaml:"freshness_yellow_days"`
+	FreshnessRedDays    int    `yaml:"freshness_red_days"`
+	MinQuotesForMedian  int    `yaml:"min_quotes_for_median"`
+	PopularityDecayDays int    `yaml:"popularity_decay_days"`
+}
+
+type ExportConfig struct {
+	TempDir     string        `yaml:"temp_dir"`
+	MaxFileAge  time.Duration `yaml:"max_file_age"`
+	CompanyName string        `yaml:"company_name"`
+}
+
+type AlertsConfig struct {
+	Enabled                  bool          `yaml:"enabled"`
+	CheckInterval            time.Duration `yaml:"check_interval"`
+	HighDemandThreshold      int           `yaml:"high_demand_threshold"`
+	TrendingThresholdPercent int           `yaml:"trending_threshold_percent"`
+}
+
+type NotificationsConfig struct {
+	EmailEnabled bool   `yaml:"email_enabled"`
+	SMTPHost     string `yaml:"smtp_host"`
+	SMTPPort     int    `yaml:"smtp_port"`
+	SMTPUser     string `yaml:"smtp_user"`
+	SMTPPassword string `yaml:"smtp_password"`
+	FromAddress  string `yaml:"from_address"`
+}
+
+type LoggingConfig struct {
+	Level    string `yaml:"level"`
+	Format   string `yaml:"format"`
+	Output   string `yaml:"output"`
+	FilePath string `yaml:"file_path"`
+}
+
+func Load(path string) (*Config, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("reading config file: %w", err)
+	}
+
+	var cfg Config
+	if err := yaml.Unmarshal(data, &cfg); err != nil {
+		return nil, fmt.Errorf("parsing config file: %w", err)
+	}
+
+	cfg.setDefaults()
+
+	return &cfg, nil
+}
+
+func (c *Config) setDefaults() {
+	if c.Server.Host == "" {
+		c.Server.Host = "0.0.0.0"
+	}
+	if c.Server.Port == 0 {
+		c.Server.Port = 8080
+	}
+	if c.Server.Mode == "" {
+		c.Server.Mode = "release"
+	}
+	if c.Server.ReadTimeout == 0 {
+		c.Server.ReadTimeout = 30 * time.Second
+	}
+	if c.Server.WriteTimeout == 0 {
+		c.Server.WriteTimeout = 30 * time.Second
+	}
+
+	if c.Database.Port == 0 {
+		c.Database.Port = 3306
+	}
+	if c.Database.MaxOpenConns == 0 {
+		c.Database.MaxOpenConns = 25
+	}
+	if c.Database.MaxIdleConns == 0 {
+		c.Database.MaxIdleConns = 5
+	}
+	if c.Database.ConnMaxLifetime == 0 {
+		c.Database.ConnMaxLifetime = 5 * time.Minute
+	}
+
+	if c.Auth.TokenExpiry == 0 {
+		c.Auth.TokenExpiry = 24 * time.Hour
+	}
+	if c.Auth.RefreshExpiry == 0 {
+		c.Auth.RefreshExpiry = 7 * 24 * time.Hour
+	}
+
+	if c.Pricing.DefaultMethod == "" {
+		c.Pricing.DefaultMethod = "weighted_median"
+	}
+	if c.Pricing.DefaultPeriodDays == 0 {
+		c.Pricing.DefaultPeriodDays = 90
+	}
+	if c.Pricing.FreshnessGreenDays == 0 {
+		c.Pricing.FreshnessGreenDays = 30
+	}
+	if c.Pricing.FreshnessYellowDays == 0 {
+		c.Pricing.FreshnessYellowDays = 60
+	}
+	if c.Pricing.FreshnessRedDays == 0 {
+		c.Pricing.FreshnessRedDays = 90
+	}
+	if c.Pricing.MinQuotesForMedian == 0 {
+		c.Pricing.MinQuotesForMedian = 3
+	}
+
+	if c.Logging.Level == "" {
+		c.Logging.Level = "info"
+	}
+	if c.Logging.Format == "" {
+		c.Logging.Format = "json"
+	}
+	if c.Logging.Output == "" {
+		c.Logging.Output = "stdout"
+	}
+}
+
+func (c *Config) Address() string {
+	return fmt.Sprintf("%s:%d", c.Server.Host, c.Server.Port)
+}