feat: upload test record attachments per run case

Allow testers to attach evidence files to a single test case within a
specific test run, proving the test was actually executed. Attachments
are scoped to the RunCase (per-execution), not the shared case definition.

Backend:
- runCaseAttachments join table + model (CASCADE on runCase/attachment)
- POST/GET /runcaseattachments routes (reuse /attachments download+delete)

Frontend:
- new "Test record" tab in the run case detail pane with drag-and-drop
  upload, scoped to reporters; i18n for all 6 locales

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/messages/de.json

@@ -362,7 +362,13 @@
     "no_case_selected": "Kein Testfall ausgewählt",
     "case_detail": "Testfall-Details",
     "comments": "Kommentare",
-    "history": "Verlauf"
+    "history": "Verlauf",
+    "test_record": "Testnachweis",
+    "download": "Herunterladen",
+    "delete": "Löschen",
+    "click_to_upload": "Zum Hochladen klicken",
+    "or_drag_and_drop": " oder per Drag & Drop",
+    "max_file_size": "Max. Dateigröße"
   },
   "Comments": {
     "comments": "Kommentare",