feat: add platform marketing homepage with signup flow

Implement complete marketing homepage for Wizamart targeting Letzshop
vendors in Luxembourg. Includes:

- Marketing homepage with hero, pricing tiers, and add-ons
- 4-step signup wizard with Stripe card collection (30-day trial)
- Letzshop vendor lookup for shop claiming
- Platform API endpoints for pricing, vendors, and signup
- Stripe SetupIntent integration for trial with card upfront
- Database fields for Letzshop vendor identity tracking

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

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

models/database/vendor.py

@@ -57,6 +57,14 @@ class Vendor(Base, TimestampMixin):
     letzshop_csv_url_en = Column(String)  # URL for English CSV in Letzshop
     letzshop_csv_url_de = Column(String)  # URL for German CSV in Letzshop
 
+    # Letzshop Vendor Identity (for linking to Letzshop marketplace profile)
+    letzshop_vendor_id = Column(
+        String(100), unique=True, nullable=True, index=True
+    )  # Letzshop's vendor identifier
+    letzshop_vendor_slug = Column(
+        String(200), nullable=True, index=True
+    )  # Letzshop shop URL slug (e.g., "my-shop" from letzshop.lu/vendors/my-shop)
+
     # ========================================================================
     # Letzshop Feed Settings (atalanda namespace)
     # ========================================================================