fix: upsert report metadata to avoid duplicate key error on re-generation

When generating a report for the same jahr/kw, update the existing
record instead of inserting a duplicate (which caused IntegrityError).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/api/reports.py

@@ -119,7 +119,18 @@ def generate_report(
         with open(filepath, "wb") as f:
             f.write(xlsx_bytes)
 
-        # Save report metadata to DB
+        # Upsert report metadata (replace if same jahr/kw exists)
+        report = (
+            db.query(WeeklyReport)
+            .filter(WeeklyReport.jahr == jahr, WeeklyReport.kw == kw)
+            .first()
+        )
+        if report:
+            report.report_date = date.today()
+            report.report_data = report_data
+            report.report_file_path = filepath
+            report.generated_by = user.id
+        else:
             report = WeeklyReport(
                 jahr=jahr,
                 kw=kw,
@@ -128,8 +139,8 @@ def generate_report(
                 generated_by=user.id,
             )
             report.report_file_path = filepath
-
             db.add(report)
+
         db.commit()
         db.refresh(report)