feat: add email system with multi-provider support

Implements a comprehensive email system with:
- Multi-provider support (SMTP, SendGrid, Mailgun, Amazon SES)
- Database-stored templates with i18n (EN, FR, DE, LB)
- Jinja2 template rendering with variable interpolation
- Email logging for debugging and compliance
- Debug mode for development (logs instead of sending)
- Welcome email integration in signup flow

New files:
- models/database/email.py: EmailTemplate and EmailLog models
- app/services/email_service.py: Provider abstraction and service
- scripts/seed_email_templates.py: Template seeding script
- tests/unit/services/test_email_service.py: 28 unit tests
- docs/features/email-system.md: Complete documentation

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

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

app/services/platform_signup_service.py

@@ -22,12 +22,14 @@ from app.exceptions import (
     ResourceNotFoundException,
     ValidationException,
 )
+from app.services.email_service import EmailService
 from app.services.stripe_service import stripe_service
 from middleware.auth import AuthManager
 from models.database.company import Company
 from models.database.subscription import (
     SubscriptionStatus,
     TierCode,
+    TIER_LIMITS,
     VendorSubscription,
 )
 from models.database.user import User
@@ -467,6 +469,62 @@ class PlatformSignupService:
 
         return setup_intent.client_secret, stripe_customer_id
 
+    # =========================================================================
+    # Welcome Email
+    # =========================================================================
+
+    def send_welcome_email(
+        self,
+        db: Session,
+        user: User,
+        vendor: Vendor,
+        tier_code: str,
+        language: str = "fr",
+    ) -> None:
+        """
+        Send welcome email to new vendor.
+
+        Args:
+            db: Database session
+            user: User who signed up
+            vendor: Vendor that was created
+            tier_code: Selected tier code
+            language: Language for email (default: French)
+        """
+        try:
+            # Get tier name
+            tier_enum = TierCode(tier_code)
+            tier_name = TIER_LIMITS.get(tier_enum, {}).get("name", tier_code.title())
+
+            # Build login URL
+            login_url = f"https://{settings.platform_domain}/vendor/{vendor.vendor_code}/dashboard"
+
+            email_service = EmailService(db)
+            email_service.send_template(
+                template_code="signup_welcome",
+                language=language,
+                to_email=user.email,
+                to_name=f"{user.first_name} {user.last_name}",
+                variables={
+                    "first_name": user.first_name,
+                    "company_name": vendor.name,
+                    "email": user.email,
+                    "vendor_code": vendor.vendor_code,
+                    "login_url": login_url,
+                    "trial_days": settings.stripe_trial_days,
+                    "tier_name": tier_name,
+                },
+                vendor_id=vendor.id,
+                user_id=user.id,
+                related_type="signup",
+            )
+
+            logger.info(f"Welcome email sent to {user.email}")
+
+        except Exception as e:
+            # Log error but don't fail signup
+            logger.error(f"Failed to send welcome email to {user.email}: {e}")
+
     # =========================================================================
     # Signup Completion
     # =========================================================================
@@ -543,6 +601,15 @@ class PlatformSignupService:
             else datetime.now(UTC) + timedelta(days=30)
         )
 
+        # Get user for welcome email
+        user_id = session.get("user_id")
+        user = db.query(User).filter(User.id == user_id).first() if user_id else None
+
+        # Send welcome email
+        if user and vendor:
+            tier_code = session.get("tier_code", TierCode.ESSENTIAL.value)
+            self.send_welcome_email(db, user, vendor, tier_code)
+
         # Clean up session
         self.delete_session(session_id)