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/api/v1/platform/__init__.py

@@ -0,0 +1,22 @@
+# app/api/v1/platform/__init__.py
+"""
+Platform public API endpoints.
+
+These endpoints are publicly accessible (no authentication required)
+and serve the marketing homepage, pricing pages, and signup flows.
+"""
+
+from fastapi import APIRouter
+
+from app.api.v1.platform import pricing, letzshop_vendors, signup
+
+router = APIRouter()
+
+# Public pricing and tier info
+router.include_router(pricing.router, tags=["platform-pricing"])
+
+# Letzshop vendor lookup
+router.include_router(letzshop_vendors.router, tags=["platform-vendors"])
+
+# Signup flow
+router.include_router(signup.router, tags=["platform-signup"])