fix(#85): resolve TypeScript compilation and validation issues

- Fix @IsNumber() validator on timestamp field (was @IsString() - critical security issue)
- Fix TypeScript compilation error in sortObjectKeys array handling
- Replace generic Error with UnauthorizedException and ServiceUnavailableException
- Document hardcoded workspace ID limitation in handleIncomingConnection
- Remove unused BadRequestException import

All tests passing (70/70), TypeScript compiles cleanly, linting passes.

apps/api/src/federation/dto/connection.dto.ts

@@ -4,7 +4,7 @@
  * Data Transfer Objects for federation connection API.
  */
 
-import { IsString, IsUrl, IsOptional, IsObject } from "class-validator";
+import { IsString, IsUrl, IsOptional, IsObject, IsNumber } from "class-validator";
 
 /**
  * DTO for initiating a connection
@@ -56,7 +56,7 @@ export class IncomingConnectionRequestDto {
   @IsObject()
   capabilities!: Record<string, unknown>;
 
-  @IsString()
+  @IsNumber()
   timestamp!: number;
 
   @IsString()