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/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", []),