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/models/onboarding_email.go

@@ -0,0 +1,34 @@
+package models
+
+import (
+	"time"
+)
+
+// OnboardingEmailType represents the type of onboarding email
+type OnboardingEmailType string
+
+const (
+	// OnboardingEmailNoResidence is sent when a user has not created a residence after 2 days
+	OnboardingEmailNoResidence OnboardingEmailType = "no_residence"
+	// OnboardingEmailNoTasks is sent when a user has created a residence but no tasks after 5 days
+	OnboardingEmailNoTasks OnboardingEmailType = "no_tasks"
+)
+
+// OnboardingEmail tracks sent onboarding emails per user
+// Each email type can only be sent once per user (enforced by unique constraint on user_id + email_type)
+type OnboardingEmail struct {
+	ID         uint                `gorm:"primaryKey" json:"id"`
+	UserID     uint                `gorm:"not null;index" json:"user_id"`
+	User       User                `gorm:"foreignKey:UserID" json:"user,omitempty"`
+	EmailType  OnboardingEmailType `gorm:"type:varchar(50);not null;index" json:"email_type"`
+	SentAt     time.Time           `gorm:"not null" json:"sent_at"`
+	OpenedAt   *time.Time          `json:"opened_at"`
+	TrackingID string              `gorm:"type:varchar(64);uniqueIndex" json:"tracking_id"`
+	CreatedAt  time.Time           `json:"created_at"`
+	UpdatedAt  time.Time           `json:"updated_at"`
+}
+
+// TableName returns the table name for OnboardingEmail
+func (OnboardingEmail) TableName() string {
+	return "onboarding_emails"
+}