feat: Implement account deletion (mark for removal) feature

- Added `marked_for_deletion` and `marked_for_deletion_at` fields to User model (Python and TypeScript) with serialization updates
- Created POST /api/user/mark-for-deletion endpoint with JWT auth, error handling, and SSE event trigger
- Blocked login and password reset for marked users; added new error codes ACCOUNT_MARKED_FOR_DELETION and ALREADY_MARKED
- Updated UserProfile.vue with "Delete My Account" button, confirmation modal (email input), loading state, success/error modals, and sign-out/redirect logic
- Synced error codes and model fields between backend and frontend
- Added and updated backend and frontend tests to cover all flows and edge cases
- All Acceptance Criteria from the spec are complete and verified

.vscode/launch.json

@@ -37,6 +37,31 @@
       "request": "launch",
       "url": "https://localhost:5173", // or your Vite dev server port
       "webRoot": "${workspaceFolder}/frontend/vue-app"
+    },
+    {
+      "name": "Python: Backend Tests",
+      "type": "python",
+      "request": "launch",
+      "program": "${workspaceFolder}/backend/.venv/Scripts/pytest.exe",
+      "args": [
+        "tests/"
+      ],
+      "cwd": "${workspaceFolder}/backend",
+      "console": "integratedTerminal",
+      "env": {
+        "PYTHONPATH": "${workspaceFolder}/backend"
+      }
+    },
+    {
+      "name": "Vue: Frontend Tests",
+      "type": "node",
+      "request": "launch",
+      "runtimeExecutable": "npx",
+      "runtimeArgs": [
+        "vitest"
+      ],
+      "cwd": "${workspaceFolder}/frontend/vue-app",
+      "console": "integratedTerminal"
     }
   ],
   "compounds": [