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/connection.service.ts

@@ -4,7 +4,13 @@
  * Manages federation connections between instances.
  */
 
-import { Injectable, Logger, NotFoundException } from "@nestjs/common";
+import {
+  Injectable,
+  Logger,
+  NotFoundException,
+  UnauthorizedException,
+  ServiceUnavailableException,
+} from "@nestjs/common";
 import { HttpService } from "@nestjs/axios";
 import { FederationConnectionStatus, Prisma } from "@prisma/client";
 import { PrismaService } from "../prisma/prisma.service";
@@ -247,7 +253,7 @@ export class ConnectionService {
     if (!validation.valid) {
       const errorMsg = validation.error ?? "Unknown error";
       this.logger.warn(`Invalid connection request from ${request.instanceId}: ${errorMsg}`);
-      throw new Error("Invalid connection request signature");
+      throw new UnauthorizedException("Invalid connection request signature");
     }
 
     // Create pending connection
@@ -284,7 +290,9 @@ export class ConnectionService {
     } catch (error: unknown) {
       this.logger.error(`Failed to fetch remote identity from ${remoteUrl}`, error);
       const errorMessage = error instanceof Error ? error.message : "Unknown error";
-      throw new Error(`Could not connect to remote instance: ${remoteUrl}: ${errorMessage}`);
+      throw new ServiceUnavailableException(
+        `Could not connect to remote instance: ${remoteUrl}: ${errorMessage}`
+      );
     }
   }