test: add platform API integration tests (67 tests)

Add comprehensive test coverage for platform marketing homepage API:
- test_pricing.py: 17 tests for tiers, add-ons, pricing endpoints
- test_letzshop_vendors.py: 22 tests for vendor lookup and claiming
- test_signup.py: 28 tests for multi-step signup flow

Fix signup.py to use correct password hashing from middleware/auth.py
and properly create Company with owner_user_id.

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

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

docs/implementation/platform-marketing-homepage.md

@@ -428,6 +428,32 @@ STRIPE_TRIAL_DAYS=30
 
 ## Testing
 
+### Automated Tests
+
+Test files located in `tests/integration/api/v1/platform/`:
+
+| File | Tests | Description |
+|------|-------|-------------|
+| `test_pricing.py` | 17 | Tier and add-on pricing endpoints |
+| `test_letzshop_vendors.py` | 22 | Vendor lookup and listing endpoints |
+| `test_signup.py` | 28 | Multi-step signup flow |
+
+**Run tests:**
+```bash
+pytest tests/integration/api/v1/platform/ -v
+```
+
+**Test categories:**
+- `TestPlatformPricingAPI` - GET /tiers, /addons, /pricing
+- `TestLetzshopVendorLookupAPI` - Vendor lookup and claiming
+- `TestLetzshopSlugExtraction` - URL parsing edge cases
+- `TestSignupStartAPI` - Signup initiation
+- `TestClaimVendorAPI` - Letzshop vendor claiming
+- `TestCreateAccountAPI` - Account creation
+- `TestSetupPaymentAPI` - Stripe SetupIntent
+- `TestCompleteSignupAPI` - Signup completion
+- `TestSignupFullFlow` - End-to-end flow
+
 ### Manual Testing
 
 1. **Homepage:** Visit `http://localhost:8000/`