feat: add admin inventory management (Phase 1)

- Add admin API endpoints for inventory management
- Add inventory page with vendor selector and filtering
- Add admin schemas for cross-vendor inventory operations
- Support digital products with unlimited inventory
- Add integration tests for admin inventory API
- Add inventory management guide documentation

Mirrors vendor inventory functionality with admin-level access.

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

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

models/schema/inventory.py

@@ -96,3 +96,107 @@ class InventorySummaryResponse(BaseModel):
     gtin: str
     total_quantity: int
     locations: list[InventoryLocationResponse]
+
+
+# ============================================================================
+# Admin Inventory Schemas
+# ============================================================================
+
+
+class AdminInventoryCreate(BaseModel):
+    """Admin version of inventory create - requires explicit vendor_id."""
+
+    vendor_id: int = Field(..., description="Target vendor ID")
+    product_id: int = Field(..., description="Product ID in vendor catalog")
+    location: str = Field(..., description="Storage location")
+    quantity: int = Field(..., description="Exact inventory quantity", ge=0)
+
+
+class AdminInventoryAdjust(BaseModel):
+    """Admin version of inventory adjust - requires explicit vendor_id."""
+
+    vendor_id: int = Field(..., description="Target vendor ID")
+    product_id: int = Field(..., description="Product ID in vendor catalog")
+    location: str = Field(..., description="Storage location")
+    quantity: int = Field(
+        ..., description="Quantity to add (positive) or remove (negative)"
+    )
+    reason: str | None = Field(None, description="Reason for adjustment")
+
+
+class AdminInventoryItem(BaseModel):
+    """Inventory item with vendor info for admin list view."""
+
+    model_config = ConfigDict(from_attributes=True)
+
+    id: int
+    product_id: int
+    vendor_id: int
+    vendor_name: str | None = None
+    vendor_code: str | None = None
+    product_title: str | None = None
+    product_sku: str | None = None
+    location: str
+    quantity: int
+    reserved_quantity: int
+    available_quantity: int
+    gtin: str | None = None
+    created_at: datetime
+    updated_at: datetime
+
+
+class AdminInventoryListResponse(BaseModel):
+    """Cross-vendor inventory list for admin."""
+
+    inventories: list[AdminInventoryItem]
+    total: int
+    skip: int
+    limit: int
+    vendor_filter: int | None = None
+    location_filter: str | None = None
+
+
+class AdminInventoryStats(BaseModel):
+    """Inventory statistics for admin dashboard."""
+
+    total_entries: int
+    total_quantity: int
+    total_reserved: int
+    total_available: int
+    low_stock_count: int
+    vendors_with_inventory: int
+    unique_locations: int
+
+
+class AdminLowStockItem(BaseModel):
+    """Low stock item for admin alerts."""
+
+    id: int
+    product_id: int
+    vendor_id: int
+    vendor_name: str | None = None
+    product_title: str | None = None
+    location: str
+    quantity: int
+    reserved_quantity: int
+    available_quantity: int
+
+
+class AdminVendorWithInventory(BaseModel):
+    """Vendor with inventory entries."""
+
+    id: int
+    name: str
+    vendor_code: str
+
+
+class AdminVendorsWithInventoryResponse(BaseModel):
+    """Response for vendors with inventory list."""
+
+    vendors: list[AdminVendorWithInventory]
+
+
+class AdminInventoryLocationsResponse(BaseModel):
+    """Response for unique inventory locations."""
+
+    locations: list[str]