refactor: move platform API database queries to service layer

- Create platform_signup_service.py for signup flow operations
- Create platform_pricing_service.py for pricing data operations
- Refactor signup.py, pricing.py, letzshop_vendors.py to use services
- Add # public markers to all platform endpoints
- Update tests for correct mock paths and status codes

Fixes architecture validation errors (API-002, API-003, SVC-006).

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

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

app/services/platform_pricing_service.py

@@ -0,0 +1,94 @@
+# app/services/platform_pricing_service.py
+"""
+Platform pricing service.
+
+Handles database operations for subscription tiers and add-on products.
+"""
+
+from sqlalchemy.orm import Session
+
+from models.database.subscription import (
+    AddOnProduct,
+    SubscriptionTier,
+    TIER_LIMITS,
+    TierCode,
+)
+
+
+class PlatformPricingService:
+    """Service for handling pricing data operations."""
+
+    def get_public_tiers(self, db: Session) -> list[SubscriptionTier]:
+        """
+        Get all public subscription tiers from the database.
+
+        Returns:
+            List of active, public subscription tiers ordered by display_order
+        """
+        return (
+            db.query(SubscriptionTier)
+            .filter(
+                SubscriptionTier.is_active == True,
+                SubscriptionTier.is_public == True,
+            )
+            .order_by(SubscriptionTier.display_order)
+            .all()
+        )
+
+    def get_tier_by_code(self, db: Session, tier_code: str) -> SubscriptionTier | None:
+        """
+        Get a specific tier by code from the database.
+
+        Args:
+            db: Database session
+            tier_code: The tier code to look up
+
+        Returns:
+            SubscriptionTier if found, None otherwise
+        """
+        return (
+            db.query(SubscriptionTier)
+            .filter(
+                SubscriptionTier.code == tier_code,
+                SubscriptionTier.is_active == True,
+            )
+            .first()
+        )
+
+    def get_tier_from_hardcoded(self, tier_code: str) -> dict | None:
+        """
+        Get tier limits from hardcoded TIER_LIMITS.
+
+        Args:
+            tier_code: The tier code to look up
+
+        Returns:
+            Dict with tier limits if valid code, None otherwise
+        """
+        try:
+            tier_enum = TierCode(tier_code)
+            limits = TIER_LIMITS[tier_enum]
+            return {
+                "tier_enum": tier_enum,
+                "limits": limits,
+            }
+        except ValueError:
+            return None
+
+    def get_active_addons(self, db: Session) -> list[AddOnProduct]:
+        """
+        Get all active add-on products from the database.
+
+        Returns:
+            List of active add-on products ordered by category and display_order
+        """
+        return (
+            db.query(AddOnProduct)
+            .filter(AddOnProduct.is_active == True)
+            .order_by(AddOnProduct.category, AddOnProduct.display_order)
+            .all()
+        )
+
+
+# Singleton instance
+platform_pricing_service = PlatformPricingService()