Add onboarding email campaign system with post-verification welcome email

Implements automated onboarding emails to encourage user engagement:
- Post-verification welcome email with 5 tips (sent after email verification)
- "No Residence" email (2+ days after registration with no property)
- "No Tasks" email (5+ days after first residence with no tasks)

Key features:
- Each onboarding email type sent only once per user (enforced by unique constraint)
- Email open tracking via tracking pixel endpoint
- Daily scheduled job at 10:00 AM UTC to process eligible users
- Admin panel UI for viewing sent emails, stats, and manual sending
- Admin can send any email type to users from the user detail Testing section

New files:
- internal/models/onboarding_email.go - Database model with tracking
- internal/services/onboarding_email_service.go - Business logic and eligibility queries
- internal/handlers/tracking_handler.go - Email open tracking endpoint
- internal/admin/handlers/onboarding_handler.go - Admin API endpoints
- admin/src/app/(dashboard)/onboarding-emails/ - Admin UI pages

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

internal/config/config.go

@@ -30,6 +30,7 @@ type ServerConfig struct {
 	AllowedHosts []string
 	Timezone     string
 	StaticDir    string // Directory for static landing page files
+	BaseURL      string // Public base URL for email tracking links (e.g., https://casera.app)
 }
 
 type DatabaseConfig struct {
@@ -146,6 +147,7 @@ func Load() (*Config, error) {
 			AllowedHosts: strings.Split(viper.GetString("ALLOWED_HOSTS"), ","),
 			Timezone:     viper.GetString("TIMEZONE"),
 			StaticDir:    viper.GetString("STATIC_DIR"),
+			BaseURL:      viper.GetString("BASE_URL"),
 		},
 		Database: dbConfig,
 		Redis: RedisConfig{
@@ -214,6 +216,7 @@ func setDefaults() {
 	viper.SetDefault("ALLOWED_HOSTS", "localhost,127.0.0.1")
 	viper.SetDefault("TIMEZONE", "UTC")
 	viper.SetDefault("STATIC_DIR", "/app/static")
+	viper.SetDefault("BASE_URL", "https://casera.app")
 
 	// Database defaults
 	viper.SetDefault("DB_HOST", "localhost")