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/services/excel_import.py

@@ -1,5 +1,6 @@
 """Excel import service — maps parsed rows to Product library."""
 from dataclasses import dataclass, field
+from pathlib import PurePosixPath
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.services.product_service import (
@@ -8,6 +9,13 @@ from app.services.product_service import (
 )
 
 
+def _stem_lower(name: str | None) -> str:
+    """Return the lowercased stem (no extension) of a filename."""
+    if not name:
+        return ""
+    return PurePosixPath(name).stem.lower()
+
+
 @dataclass
 class PreviewResult:
     """Read-only preview: annotates rows without creating anything."""
@@ -18,6 +26,8 @@ class PreviewResult:
     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] = field(default_factory=list)
 
 
@@ -118,8 +128,8 @@ async def preview_excel_rows(
     """
     result = PreviewResult()
     # Track unique identifiers we've already resolved in this batch
-    # key = lower(baureihe) or pim_id  →  (product_exists, product_id_str | None, has_step, first_row_index)
-    seen: dict[str, tuple[bool, str | None, bool, int]] = {}
+    # key = lower(baureihe) or pim_id  →  (product_exists, product_id_str | None, has_step, first_row_index, name_cad_modell_stem)
+    seen: dict[str, tuple[bool, str | None, bool, int, str]] = {}
 
     for row in parsed_rows:
         pim_id = row.get("pim_id")
@@ -127,6 +137,14 @@ async def preview_excel_rows(
         row_index = row.get("row_index", 0)
         row["category_key"] = row.get("category_key") or category_key
 
+        # Default conflict fields
+        row["step_conflict"] = False
+        row["step_conflict_existing_name"] = None
+        row["step_conflict_excel_name"] = None
+        row["cad_name_conflict"] = False
+        row["cad_name_conflict_other_name"] = None
+        row["cad_name_conflict_row"] = None
+
         # Must have at least one identifier
         if not pim_id and not produkt_baureihe:
             row["product_exists"] = False
@@ -139,13 +157,24 @@ async def preview_excel_rows(
         # Build a cache key
         cache_key = (produkt_baureihe or "").lower() or pim_id or ""
 
+        excel_cad_name = row.get("name_cad_modell")
+        excel_cad_stem = _stem_lower(excel_cad_name)
+
         if cache_key in seen:
-            exists, pid, has_step, first_row = seen[cache_key]
+            exists, pid, has_step, first_row, first_cad_stem = seen[cache_key]
             row["product_exists"] = exists
             row["product_id"] = pid
             row["has_step"] = has_step
             row["is_duplicate"] = True
             row["duplicate_of_row"] = first_row
+
+            # Intra-Excel conflict: same product key, different name_cad_modell
+            if excel_cad_stem and first_cad_stem and excel_cad_stem != first_cad_stem:
+                row["cad_name_conflict"] = True
+                row["cad_name_conflict_other_name"] = first_cad_stem
+                row["cad_name_conflict_row"] = first_row
+                result.cad_name_conflict_count += 1
+
             result.duplicate_count += 1
             continue
 
@@ -156,17 +185,26 @@ async def preview_excel_rows(
             row["product_exists"] = True
             row["product_id"] = str(product.id)
             row["has_step"] = has_step
-            seen[cache_key] = (True, str(product.id), has_step, row_index)
+            seen[cache_key] = (True, str(product.id), has_step, row_index, excel_cad_stem)
             result.existing_product_count += 1
             if has_step:
                 result.has_step_count += 1
             else:
                 result.no_step_count += 1
+
+            # STEP conflict: product already has a different STEP file
+            if has_step and excel_cad_stem and product.cad_file:
+                existing_stem = _stem_lower(product.cad_file.original_name)
+                if existing_stem and existing_stem != excel_cad_stem:
+                    row["step_conflict"] = True
+                    row["step_conflict_existing_name"] = existing_stem
+                    row["step_conflict_excel_name"] = excel_cad_stem
+                    result.step_conflict_count += 1
         else:
             row["product_exists"] = False
             row["product_id"] = None
             row["has_step"] = False
-            seen[cache_key] = (False, None, False, row_index)
+            seen[cache_key] = (False, None, False, row_index, excel_cad_stem)
             result.new_product_count += 1
             result.no_step_count += 1
 
@@ -176,4 +214,13 @@ async def preview_excel_rows(
             f"{result.duplicate_count} duplicate Produkt-Baureihe row(s) detected — "
             "these are pre-unchecked. Only one row per product will be imported."
         )
+    if result.step_conflict_count > 0:
+        result.warnings.append(
+            f"{result.step_conflict_count} product(s) already have a different STEP file linked — "
+            "importing will not replace the existing STEP file automatically."
+        )
+    if result.cad_name_conflict_count > 0:
+        result.warnings.append(
+            f"{result.cad_name_conflict_count} row(s) reference the same product with a different CAD model name."
+        )
     return result