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/subscription.py

@@ -452,6 +452,9 @@ class VendorSubscription(Base, TimestampMixin):
     # Trial info
     trial_ends_at = Column(DateTime(timezone=True), nullable=True)
 
+    # Card collection tracking (for trials that require card upfront)
+    card_collected_at = Column(DateTime(timezone=True), nullable=True)
+
     # Usage counters (reset each billing period)
     orders_this_period = Column(Integer, default=0, nullable=False)
     orders_limit_reached_at = Column(DateTime(timezone=True), nullable=True)