feat: add inventory CSV import with warehouse/bin locations

- Add warehouse and bin_location columns to Inventory model
- Create inventory_import_service for bulk TSV/CSV import
- Add POST /api/v1/admin/inventory/import endpoint
- Add Import button and modal to inventory admin page
- Support both single-unit rows and explicit QUANTITY column

File format: BIN, EAN, PRODUCT (optional), QUANTITY (optional)
Products matched by GTIN/EAN, unmatched items reported.

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

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

models/database/inventory.py

@@ -1,4 +1,16 @@
 # models/database/inventory.py
+"""
+Inventory model for tracking stock at warehouse/bin locations.
+
+Each entry represents a quantity of a product at a specific bin location
+within a warehouse. Products can be scattered across multiple bins.
+
+Example:
+    Warehouse: "strassen"
+    Bin: "SA-10-02"
+    Product: GTIN 4007817144145
+    Quantity: 3
+"""
 
 from sqlalchemy import Column, ForeignKey, Index, Integer, String, UniqueConstraint
 from sqlalchemy.orm import relationship
@@ -14,11 +26,17 @@ class Inventory(Base, TimestampMixin):
     product_id = Column(Integer, ForeignKey("products.id"), nullable=False, index=True)
     vendor_id = Column(Integer, ForeignKey("vendors.id"), nullable=False, index=True)
 
-    location = Column(String, 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)
 
-    # Optional: Keep GTIN for reference/reporting
+    # Keep GTIN for reference/reporting (matches Product.gtin)
     gtin = Column(String, index=True)
 
     # Relationships
@@ -28,10 +46,10 @@ class Inventory(Base, TimestampMixin):
     # Constraints
     __table_args__ = (
         UniqueConstraint(
-            "product_id", "location", name="uq_inventory_product_location"
+            "product_id", "warehouse", "bin_location", name="uq_inventory_product_warehouse_bin"
         ),
         Index("idx_inventory_vendor_product", "vendor_id", "product_id"),
-        Index("idx_inventory_product_location", "product_id", "location"),
+        Index("idx_inventory_warehouse_bin", "warehouse", "bin_location"),
     )
 
     def __repr__(self):