fix: resolve dart analyze failures in chaos_monkey_test.dart (#458)

## Summary

Fixes CI failures introduced by PR #455 (chaos monkey backend test).

The `dart analyze --fatal-infos` step in CI was failing because `test/backend/chaos_monkey_test.dart` had:

- **`avoid_print`** (5 instances): replaced `print(...)` with `stdout.writeln(...)` — `dart:io` is already imported
- **`avoid_redundant_argument_values`**: removed redundant `''` from `_env('CHAOS_SEED', '')` since `''` is the parameter default
- **`dart format`**: applied formatter fixes (trailing commas, line wrapping for long `connectToServer` calls)

## Verification

```
$ nix develop --command bash -c "fvm dart analyze --fatal-infos"
Analyzing 456...
No issues found!

$ nix develop --command bash -c "fvm dart format --output=none --set-exit-if-changed test/backend/chaos_monkey_test.dart"
Formatted 1 file (0 changed) in 0.01 seconds.
```

Closes #456

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Co-authored-by: Thomas SharedInbox <sharedinbox@thomas-guettler.de>
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/458

.forgejo/workflows/chaos-monkey.yml

@@ -0,0 +1,20 @@
+name: Chaos Monkey
+
+on:
+  schedule:
+    - cron: '0 3 * * *'
+  workflow_dispatch:
+
+jobs:
+  chaos-monkey-backend:
+    name: Chaos Monkey (backend)
+    runs-on: ubuntu-latest
+    timeout-minutes: 60
+    steps:
+      - uses: actions/checkout@v4
+      - name: Setup Dagger Remote Engine
+        env:
+          SOPS_AGE_KEY: ${{ secrets.SOPS_AGE_KEY }}
+        run: scripts/setup_dagger_remote.sh
+      - name: Run backend chaos monkey
+        run: task chaos-monkey-backend