fix: return full maintenance scheduler results

[itzptk]

Summary

• Type BackgroundScheduler maintenance methods with the shared IPC contracts they already return.
• Preserve full manual metadata sync results (success, output, syncedAt) and cleanup job completion events through the scheduler boundary.
• Add scheduler coverage that the manual wrappers return the full metadata sync and cleanup payloads while still persisting timestamps.

Closes #34

Verification

• RED: npx tsc --noEmit -p tsconfig.electron.json failed before the fix with the target electron/main.ts(452,5) and electron/main.ts(455,5) TS2322 return-type errors, plus the new compile-time regression assertions failed because scheduler methods returned narrowed shapes.
• ELECTRON_OVERRIDE_DIST_PATH=/tmp npm run test:node -- electron/services/background-scheduler.test.ts → 8 tests passed.
• ELECTRON_OVERRIDE_DIST_PATH=/tmp npm run test:node → 15 files / 117 tests passed.
• ELECTRON_OVERRIDE_DIST_PATH=/tmp npm run build:electron → tsup build success.
• ELECTRON_OVERRIDE_DIST_PATH=/tmp npm run build → Angular production build and Electron build succeeded.
• npx tsc --noEmit -p tsconfig.electron.json still exits 2 due pre-existing unrelated Electron type errors, but the target runManualMetadataSync / runManualCleanup TS2322 errors are absent after the fix; remaining related pre-existing error observed: electron/services/background-scheduler.test.ts(91,5): error TS2349.