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>

app/core/config.py

@@ -123,7 +123,7 @@ class Settings(BaseSettings):
     stripe_secret_key: str = ""
     stripe_publishable_key: str = ""
     stripe_webhook_secret: str = ""
-    stripe_trial_days: int = 14
+    stripe_trial_days: int = 30  # 1-month free trial (card collected upfront but not charged)
 
     # =============================================================================
     # DEMO/SEED DATA CONFIGURATION