feat: duplicate product detection — STEP conflict warnings on Excel import and CAD upload

- Excel preview detects when a product already has a different STEP file linked
- Excel preview detects intra-Excel conflicts (same product, different CAD model names)
- Product STEP upload warns when replacing an existing file and shows render count
- All warnings are non-blocking (amber badges, toast warnings)
- LEARNINGS.md: all open items resolved

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/domains/products/service.py

@@ -2,6 +2,7 @@
 import uuid
 from sqlalchemy import select, func, update as sql_update
 from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm import selectinload
 
 from app.domains.products.models import Product
 
@@ -48,7 +49,9 @@ async def lookup_product(
     """
     if produkt_baureihe:
         result = await db.execute(
-            select(Product).where(
+            select(Product)
+            .options(selectinload(Product.cad_file))
+            .where(
                 func.lower(Product.produkt_baureihe) == produkt_baureihe.lower(),
                 Product.is_active.is_(True),
             )
@@ -61,7 +64,9 @@ async def lookup_product(
 
     if pim_id:
         result = await db.execute(
-            select(Product).where(Product.pim_id == pim_id, Product.is_active.is_(True))
+            select(Product)
+            .options(selectinload(Product.cad_file))
+            .where(Product.pim_id == pim_id, Product.is_active.is_(True))
         )
         return result.scalar_one_or_none()