feature(devices,topology,tz): devices bulk-add, duplicate safety, and firmware/timezone fixes

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.

frontend/src/types/index.ts

@@ -190,6 +190,8 @@ export interface TopologyDevice {
   device_type: DeviceType;
   status: DeviceStatus;
   ros_version?: string;
+  /** Cached /ip/address rows from the device (set by backend sync). */
+  ip_addresses_jsonb?: { address: string; interface?: string; dynamic?: boolean }[];
 }
 
 export interface TopologyLink {