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/db/auth.go

@@ -0,0 +1,137 @@
+package db
+
+import (
+	"context"
+	"time"
+
+	"github.com/jackc/pgx/v5"
+)
+
+func (r *PGRepository) GetUserByEmail(ctx context.Context, email string) (*UserRecord, error) {
+	var user UserRecord
+	var name, passwordHash *string
+	var lastLoginAt *time.Time
+
+	err := r.pool.QueryRow(ctx, `
+		SELECT id, email, name, password_hash, email_verified, provider, role, created_at, last_login_at
+		FROM users 
+		WHERE email = $1
+	`, email).Scan(
+		&user.ID, &user.Email, &name, &passwordHash,
+		&user.EmailVerified, &user.Provider, &user.Role,
+		&user.CreatedAt, &lastLoginAt,
+	)
+	if err == pgx.ErrNoRows {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	user.Name = name
+	user.PasswordHash = passwordHash
+	user.LastLoginAt = lastLoginAt
+	return &user, nil
+}
+
+func (r *PGRepository) GetUserByID(ctx context.Context, userID string) (*UserRecord, error) {
+	var user UserRecord
+	var name, passwordHash *string
+	var lastLoginAt *time.Time
+
+	err := r.pool.QueryRow(ctx, `
+		SELECT id, email, name, password_hash, email_verified, provider, role, created_at, last_login_at
+		FROM users 
+		WHERE id = $1
+	`, userID).Scan(
+		&user.ID, &user.Email, &name, &passwordHash,
+		&user.EmailVerified, &user.Provider, &user.Role,
+		&user.CreatedAt, &lastLoginAt,
+	)
+	if err == pgx.ErrNoRows {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	user.Name = name
+	user.PasswordHash = passwordHash
+	user.LastLoginAt = lastLoginAt
+	return &user, nil
+}
+
+func (r *PGRepository) CreateUser(ctx context.Context, email, passwordHash, name, provider, role string) (*UserRecord, error) {
+	var user UserRecord
+	var lastLoginAt *time.Time
+
+	err := r.pool.QueryRow(ctx, `
+		INSERT INTO users (email, password_hash, name, provider, role, email_verified)
+		VALUES ($1, $2, $3, $4, $5, false)
+		RETURNING id, email, name, password_hash, email_verified, provider, role, created_at, last_login_at
+	`, email, nullableString(passwordHash), nullableString(name), provider, role).Scan(
+		&user.ID, &user.Email, &user.Name, &user.PasswordHash,
+		&user.EmailVerified, &user.Provider, &user.Role,
+		&user.CreatedAt, &lastLoginAt,
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	user.LastLoginAt = lastLoginAt
+	return &user, nil
+}
+
+func (r *PGRepository) UpdateLastLogin(ctx context.Context, userID string) error {
+	_, err := r.pool.Exec(ctx, `
+		UPDATE users SET last_login_at = NOW() WHERE id = $1
+	`, userID)
+	return err
+}
+
+func (r *PGRepository) MarkEmailVerified(ctx context.Context, userID string) error {
+	_, err := r.pool.Exec(ctx, `
+		UPDATE users SET email_verified = true WHERE id = $1
+	`, userID)
+	return err
+}
+
+func (r *PGRepository) CreateMagicLink(ctx context.Context, token, userID, email string, expiresAt time.Time) error {
+	_, err := r.pool.Exec(ctx, `
+		INSERT INTO magic_links (token, user_id, email, expires_at)
+		VALUES ($1, $2, $3, $4)
+	`, token, userID, email, expiresAt)
+	return err
+}
+
+func (r *PGRepository) GetMagicLink(ctx context.Context, token string) (*MagicLinkRecord, error) {
+	var ml MagicLinkRecord
+	err := r.pool.QueryRow(ctx, `
+		SELECT token, user_id, email, used, expires_at, created_at
+		FROM magic_links 
+		WHERE token = $1
+	`, token).Scan(
+		&ml.Token, &ml.UserID, &ml.Email, &ml.Used, &ml.ExpiresAt, &ml.CreatedAt,
+	)
+	if err == pgx.ErrNoRows {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+	return &ml, nil
+}
+
+func (r *PGRepository) MarkMagicLinkUsed(ctx context.Context, token string) error {
+	_, err := r.pool.Exec(ctx, `
+		UPDATE magic_links SET used = true WHERE token = $1
+	`, token)
+	return err
+}
+
+func nullableString(s string) interface{} {
+	if s == "" {
+		return nil
+	}
+	return s
+}