[Tests] SettingEndpoints — add error and invalid-input scenarios

[fboucheros]

What to build

SettingEndpointsTests.cs has only 4 happy-path tests. Add coverage for error and edge-case scenarios.

Endpoints to cover:

• GET /api/settings/
• POST /api/settings/SaveSettings
• GET /api/settings/GetNextReadingNotesCounter

Acceptance criteria

• Test: GetSettings returns sensible defaults when no settings are persisted
• Test: SaveSettings with invalid/missing required fields returns a 400
• Test: GetNextReadingNotesCounter with a non-existent or corrupt counter handles gracefully
• All new tests pass in CI

Blocked by

None — can start immediately.

Priority

🟡 Important — currently only happy-path covered