refactor: migrate modules from re-exports to canonical implementations

Move actual code implementations into module directories:
- orders: 5 services, 4 models, order/invoice schemas
- inventory: 3 services, 2 models, 30+ schemas
- customers: 3 services, 2 models, customer schemas
- messaging: 3 services, 2 models, message/notification schemas
- monitoring: background_tasks_service
- marketplace: 5+ services including letzshop submodule
- dev_tools: code_quality_service, test_runner_service
- billing: billing_service
- contracts: definition.py

Legacy files in app/services/, models/database/, models/schema/
now re-export from canonical module locations for backwards
compatibility. Architecture validator passes with 0 errors.

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

models/database/inventory.py

@@ -1,61 +1,19 @@
 # models/database/inventory.py
 """
-Inventory model for tracking stock at warehouse/bin locations.
+LEGACY LOCATION - Re-exports from module for backwards compatibility.
 
-Each entry represents a quantity of a product at a specific bin location
-within a warehouse. Products can be scattered across multiple bins.
+The canonical implementation is now in:
+    app/modules/inventory/models/inventory.py
 
-Example:
-    Warehouse: "strassen"
-    Bin: "SA-10-02"
-    Product: GTIN 4007817144145
-    Quantity: 3
+This file exists to maintain backwards compatibility with code that
+imports from the old location. All new code should import directly
+from the module:
+
+    from app.modules.inventory.models import Inventory
 """
 
-from sqlalchemy import Column, ForeignKey, Index, Integer, String, UniqueConstraint
-from sqlalchemy.orm import relationship
+from app.modules.inventory.models.inventory import Inventory
 
-from app.core.database import Base
-from models.database.base import TimestampMixin
-
-
-class Inventory(Base, TimestampMixin):
-    __tablename__ = "inventory"
-
-    id = Column(Integer, primary_key=True, index=True)
-    product_id = Column(Integer, ForeignKey("products.id"), nullable=False, index=True)
-    vendor_id = Column(Integer, ForeignKey("vendors.id"), nullable=False, index=True)
-
-    # Location: warehouse + bin
-    warehouse = Column(String, nullable=False, default="strassen", index=True)
-    bin_location = Column(String, nullable=False, index=True)  # e.g., "SA-10-02"
-
-    # Legacy field - kept for backward compatibility, will be removed
-    location = Column(String, index=True)
-
-    quantity = Column(Integer, nullable=False, default=0)
-    reserved_quantity = Column(Integer, default=0)
-
-    # Keep GTIN for reference/reporting (matches Product.gtin)
-    gtin = Column(String, index=True)
-
-    # Relationships
-    product = relationship("Product", back_populates="inventory_entries")
-    vendor = relationship("Vendor")
-
-    # Constraints
-    __table_args__ = (
-        UniqueConstraint(
-            "product_id", "warehouse", "bin_location", name="uq_inventory_product_warehouse_bin"
-        ),
-        Index("idx_inventory_vendor_product", "vendor_id", "product_id"),
-        Index("idx_inventory_warehouse_bin", "warehouse", "bin_location"),
-    )
-
-    def __repr__(self):
-        return f"<Inventory(product_id={self.product_id}, location='{self.location}', quantity={self.quantity})>"
-
-    @property
-    def available_quantity(self):
-        """Calculate available quantity (total - reserved)."""
-        return max(0, self.quantity - self.reserved_quantity)
+__all__ = [
+    "Inventory",
+]