feat(core): consolidate auth service into backend and implement stripe billing

This commit performs a major architectural refactor by migrating the standalone `auth-service` into the main `backend` application, enabling a unified codebase and simplified deployment. It also introduces comprehensive Stripe billing support and a new administrative dashboard.

Key changes:
- **Architecture**: Deleted `apps/auth-service` and integrated its functionality (JWT, magic links, OAuth, user management) into `apps/backend`.
- **Billing**: Added Stripe integration to `backend`, supporting both monthly and yearly subscription cycles with automatic plan entitlement enforcement (e.g., location limits).
- **Admin Dashboard**: Implemented a new administrative service and API endpoints to manage tenants, users, and view platform-wide statistics.
- **Frontend**:
    - Added a new pricing page with monthly/yearly toggle and comparison table.
    - Integrated Stripe and Sentry for payments and error tracking.
    - Improved dashboard UX/UI and added i18n support for new features.
    - Enhanced the public booking flow with better validation and contact form integration.
- **Database**: Added migrations for users, magic links, password resets, OAuth states, admin audit logs, and refresh tokens.
- **DevOps**: Updated environment configurations for Railway and Vercel, and streamlined the project's `package.json` scripts.

apps/backend/internal/domain/models.go

@@ -146,16 +146,32 @@ type CreateBookingResponse struct {
 type PlanEntitlements struct {
 	MaxLocations      int  `json:"maxLocations"`
 	MaxStaff          int  `json:"maxStaff"`
+	MaxBookingsMonth  int  `json:"maxBookingsMonth"`  // -1 = unlimited
 	EmailReminders    bool `json:"emailReminders"`
 	AdvancedReporting bool `json:"advancedReporting"`
 	WidgetEmbedding   bool `json:"widgetEmbedding"`
 	UmamiTracking     bool `json:"umamiTracking"`
+	APIAccess         bool `json:"apiAccess"`
+	DedicatedManager  bool `json:"dedicatedManager"`
+}
+
+type PlanPricing struct {
+	MonthlyAmountCents int    `json:"monthlyAmountCents"`
+	YearlyAmountCents  int    `json:"yearlyAmountCents"`
+	MonthlyFormatted   string `json:"monthlyFormatted"`
+	YearlyFormatted    string `json:"yearlyFormatted"`
+	YearlySavings      string `json:"yearlySavings"`
+	YearlySavingsPercent int  `json:"yearlySavingsPercent"`
 }
 
 type PlanDisplayPrice struct {
-	Currency    string `json:"currency"`
-	AmountCents int    `json:"amountCents"`
-	Formatted   string `json:"formatted"`
+	Currency            string `json:"currency"`
+	AmountCents         int    `json:"amountCents"`
+	Formatted           string `json:"formatted"`
+	YearlyAmountCents   int    `json:"yearlyAmountCents,omitempty"`
+	YearlyFormatted     string `json:"yearlyFormatted,omitempty"`
+	YearlySavings       string `json:"yearlySavings,omitempty"`
+	YearlySavingsPercent int    `json:"yearlySavingsPercent,omitempty"`
 }
 
 type SubscriptionSnapshot struct {
@@ -182,17 +198,22 @@ type SubscriptionSnapshot struct {
 }
 
 type CheckoutSessionRequest struct {
-	PlanCode string `json:"planCode"`
-	Currency string `json:"currency,omitempty"`
+	PlanCode        string `json:"planCode"`
+	Currency        string `json:"currency,omitempty"`
+	BillingInterval string `json:"billingInterval,omitempty"` // "monthly" or "yearly", defaults to "monthly"
 }
 
 type CheckoutLaunchResponse struct {
-	PriceID            string            `json:"priceId"`
+	// Stripe checkout
+	CheckoutURL        string            `json:"checkoutUrl,omitempty"`
+	// Paddle checkout
+	PriceID            string            `json:"priceId,omitempty"`
 	CustomerID         string            `json:"customerId,omitempty"`
 	CustomerEmail      string            `json:"customerEmail,omitempty"`
-	SuccessRedirectURL string            `json:"successRedirectUrl"`
-	CancelRedirectURL  string            `json:"cancelRedirectUrl"`
-	CustomData         map[string]string `json:"customData"`
+	// Common
+	SuccessRedirectURL string            `json:"successRedirectUrl,omitempty"`
+	CancelRedirectURL  string            `json:"cancelRedirectUrl,omitempty"`
+	CustomData         map[string]string `json:"customData,omitempty"`
 }
 
 type PortalSessionResponse struct {
@@ -321,6 +342,61 @@ type CancelBookingRequest struct {
 	Reason string `json:"reason,omitempty"`
 }
 
+// ============================================
+// ADMIN MODELS
+// ============================================
+
+type AdminDashboardStats struct {
+	TotalTenants         int64 `json:"totalTenants"`
+	TotalUsers           int64 `json:"totalUsers"`
+	ActiveSubscriptions  int64 `json:"activeSubscriptions"`
+	TrialSubscriptions   int64 `json:"trialSubscriptions"`
+	BookingsThisMonth    int64 `json:"bookingsThisMonth"`
+	RevenueThisMonthCents int64 `json:"revenueThisMonthCents"`
+}
+
+type AdminTenantList struct {
+	Total    int            `json:"total"`
+	Page     int            `json:"page"`
+	PageSize int            `json:"pageSize"`
+	Tenants  []AdminTenant  `json:"tenants"`
+}
+
+type AdminTenant struct {
+	ID                string `json:"id"`
+	Slug              string `json:"slug"`
+	Name              string `json:"name"`
+	PlanCode          string `json:"planCode"`
+	SubscriptionStatus string `json:"subscriptionStatus"`
+	BillingProvider   string `json:"billingProvider"`
+}
+
+type AdminUserList struct {
+	Total    int          `json:"total"`
+	Page     int          `json:"page"`
+	PageSize int          `json:"pageSize"`
+	Users    []AdminUser  `json:"users"`
+}
+
+type AdminUser struct {
+	ID            string    `json:"id"`
+	Email         string    `json:"email"`
+	Name          string    `json:"name,omitempty"`
+	EmailVerified bool      `json:"emailVerified"`
+	Provider      string    `json:"provider"`
+	Role          string    `json:"role"`
+	CreatedAt     time.Time `json:"createdAt"`
+}
+
+type AdminLoginRequest struct {
+	Email string `json:"email" binding:"required,email"`
+	Key   string `json:"key" binding:"required"`
+}
+
+type UpdateUserRoleRequest struct {
+	Role string `json:"role" binding:"required,oneof=user admin superadmin"`
+}
+
 // ============================================
 // WORKING HOURS MODELS
 // ============================================