fix: Disallow building ships with build_time=0 and document that

Author: Clashsoft

src/job/job-logic.service.ts

@@ -111,9 +111,13 @@ export class JobLogicService {
           throw new BadRequestException('Ship type and fleet id are required for this job type.');
         }
         const ship = SHIP_TYPES[dto.ship as ShipTypeName] ?? notFound(dto.ship as ShipTypeName);
+        const time = this.empireLogicService.getShipTime(empire, ship);
+        if (!time) {
+          throw new BadRequestException('This ship type is not unlocked yet (build_time=0).');
+        }
         return {
           ...this.empireLogicService.getShipCost(empire, ship),
-          time: this.empireLogicService.getShipTime(empire, ship),
+          time,
         };
       }
       case JobType.TRAVEL: {

src/job/job.schema.ts

@@ -98,7 +98,9 @@ export class Job extends GlobalSchema {
   @Prop({type: String})
   @ApiPropertyOptional({
     example: 'explorer',
-    description: 'Ship type name for the job. Required for type=ship.',
+    description: 'Ship type name for the job. Required for type=ship.\n\n' +
+      'Note that if the ship type build_time variable is 0 for this empire, the ship cannot be built.\n' +
+      'In this case, a technology is required to unlock the ship type.',
     enum: SHIP_NAMES,
   })
   @ValidateIf((job, value) => value || job.type === JobType.SHIP)