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>

app/routes/admin_pages.py

@@ -24,6 +24,8 @@ Routes:
 - GET /vendors/{vendor_code}/theme → Vendor theme editor (auth required)
 - GET /users         → User management page (auth required)
 - GET /customers     → Customer management page (auth required)
+- GET /inventory     → Inventory management page (auth required)
+- GET /orders        → Orders management page (auth required)
 - GET /imports       → Import history page (auth required)
 - GET /marketplace-products → Marketplace products catalog (auth required)
 - GET /vendor-products → Vendor products catalog (auth required)
@@ -474,6 +476,54 @@ async def admin_customers_page(
     )
 
 
+# ============================================================================
+# INVENTORY MANAGEMENT ROUTES
+# ============================================================================
+
+
+@router.get("/inventory", response_class=HTMLResponse, include_in_schema=False)
+async def admin_inventory_page(
+    request: Request,
+    current_user: User = Depends(get_current_admin_from_cookie_or_header),
+    db: Session = Depends(get_db),
+):
+    """
+    Render inventory management page.
+    Shows stock levels across all vendors with filtering and adjustment capabilities.
+    """
+    return templates.TemplateResponse(
+        "admin/inventory.html",
+        {
+            "request": request,
+            "user": current_user,
+        },
+    )
+
+
+# ============================================================================
+# ORDER MANAGEMENT ROUTES
+# ============================================================================
+
+
+@router.get("/orders", response_class=HTMLResponse, include_in_schema=False)
+async def admin_orders_page(
+    request: Request,
+    current_user: User = Depends(get_current_admin_from_cookie_or_header),
+    db: Session = Depends(get_db),
+):
+    """
+    Render orders management page.
+    Shows orders across all vendors with filtering and status management.
+    """
+    return templates.TemplateResponse(
+        "admin/orders.html",
+        {
+            "request": request,
+            "user": current_user,
+        },
+    )
+
+
 # ============================================================================
 # IMPORT MANAGEMENT ROUTES
 # ============================================================================