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>

backend/models/runCaseAttachments.js

@@ -0,0 +1,27 @@
+function defineRunCaseAttachment(sequelize, DataTypes) {
+  const RunCaseAttachment = sequelize.define('RunCaseAttachment', {
+    runCaseId: {
+      type: DataTypes.INTEGER,
+      allowNull: false,
+    },
+    attachmentId: {
+      type: DataTypes.INTEGER,
+      allowNull: false,
+    },
+  });
+
+  RunCaseAttachment.associate = (models) => {
+    RunCaseAttachment.belongsTo(models.RunCase, {
+      foreignKey: 'runCaseId',
+      onDelete: 'CASCADE',
+    });
+    RunCaseAttachment.belongsTo(models.Attachment, {
+      foreignKey: 'attachmentId',
+      onDelete: 'CASCADE',
+    });
+  };
+
+  return RunCaseAttachment;
+}
+
+export default defineRunCaseAttachment;