Squash-merge of PR #28 from @denis-ev. All three review blockers addressed: encryption key management documented, string truncation replaced with skip+warn validation, and undefined return on duplicate merge fixed with proper 500. Local test stack verified against cloned prod DB — migrations, new features, and backend all clean.
Sets up Jest + ts-jest for the backend and adds 47 tests across 5
suites covering crypto utilities, OUI vendor lookup, auth middleware,
AlertService dispatch/cooldown logic, and DeviceCollector uptime parsing.