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/api/routers/products.py

@@ -470,6 +470,11 @@ async def upload_product_cad(
     if not product:
         raise HTTPException(404, detail="Product not found")
 
+    # Check for STEP replacement warnings before proceeding
+    warnings: list[str] = []
+    existing_render_count = 0
+    old_cad_file_id = product.cad_file_id
+
     content = await file.read()
     file_hash = hashlib.sha256(content).hexdigest()
 
@@ -477,6 +482,24 @@ async def upload_product_cad(
     existing_cad = await db.execute(select(CadFile).where(CadFile.file_hash == file_hash))
     cad_file = existing_cad.scalar_one_or_none()
 
+    # Detect replacement: product already has a different CAD file
+    if old_cad_file_id and (cad_file is None or cad_file.id != old_cad_file_id):
+        old_name = product.cad_file.original_name if product.cad_file else "unknown"
+        warnings.append(
+            f"Replacing existing STEP file '{old_name}' with '{file.filename}'."
+        )
+        # Count existing renders (MediaAssets) for this product
+        from app.domains.media.models import MediaAsset
+        render_count_result = await db.execute(
+            select(func.count(MediaAsset.id)).where(MediaAsset.product_id == product_id)
+        )
+        existing_render_count = render_count_result.scalar() or 0
+        if existing_render_count > 0:
+            warnings.append(
+                f"This product has {existing_render_count} existing render(s) that were "
+                "generated from the previous STEP file. They may no longer match."
+            )
+
     if cad_file is None:
         step_dir = Path(settings.upload_dir) / "step_files"
         step_dir.mkdir(parents=True, exist_ok=True)
@@ -511,6 +534,8 @@ async def upload_product_cad(
         "file_hash": file_hash,
         "status": "uploaded" if cad_file.processing_status == ProcessingStatus.pending else "already_exists",
         "product_id": str(product_id),
+        "warnings": warnings,
+        "existing_render_count": existing_render_count,
     }
 
 

backend/app/api/routers/uploads.py

@@ -39,6 +39,14 @@ class ExcelPreviewRow(BaseModel):
     has_step: bool = False
     is_duplicate: bool = False
     duplicate_of_row: int | None = None
+    # STEP conflict: existing product has a different STEP file than Excel row's name_cad_modell
+    step_conflict: bool = False
+    step_conflict_existing_name: str | None = None
+    step_conflict_excel_name: str | None = None
+    # Intra-Excel conflict: same product key appears with different name_cad_modell
+    cad_name_conflict: bool = False
+    cad_name_conflict_other_name: str | None = None
+    cad_name_conflict_row: int | None = None
 
 
 class ExcelPreviewResponse(BaseModel):
@@ -52,6 +60,8 @@ class ExcelPreviewResponse(BaseModel):
     has_step_count: int = 0
     no_step_count: int = 0
     duplicate_count: int = 0
+    step_conflict_count: int = 0
+    cad_name_conflict_count: int = 0
     warnings: list[str]
     rows: list[ExcelPreviewRow]
     column_headers: list[str] = []
@@ -145,6 +155,12 @@ async def upload_excel(
             has_step=r.get("has_step", False),
             is_duplicate=r.get("is_duplicate", False),
             duplicate_of_row=r.get("duplicate_of_row"),
+            step_conflict=r.get("step_conflict", False),
+            step_conflict_existing_name=r.get("step_conflict_existing_name"),
+            step_conflict_excel_name=r.get("step_conflict_excel_name"),
+            cad_name_conflict=r.get("cad_name_conflict", False),
+            cad_name_conflict_other_name=r.get("cad_name_conflict_other_name"),
+            cad_name_conflict_row=r.get("cad_name_conflict_row"),
         )
         for r in preview.rows
     ]
@@ -195,6 +211,8 @@ async def upload_excel(
         has_step_count=preview.has_step_count,
         no_step_count=preview.no_step_count,
         duplicate_count=preview.duplicate_count,
+        step_conflict_count=preview.step_conflict_count,
+        cad_name_conflict_count=preview.cad_name_conflict_count,
         warnings=all_warnings,
         rows=annotated_rows,
         column_headers=parsed_dict.get("column_headers", []),