feat(#285): Add input sanitization for XSS prevention

Security improvements:
- Create sanitization utility using sanitize-html library
- Add @Sanitize() and @SanitizeObject() decorators for DTOs
- Apply sanitization to vulnerable fields:
  - Connection rejection/disconnection reasons
  - Connection metadata
  - Identity linking metadata
  - Command payloads
- Remove script tags, event handlers, javascript: URLs
- Prevent data exfiltration, CSS-based XSS, SVG-based XSS

Changes:
- Add sanitize.util.ts with recursive sanitization functions
- Add sanitize.decorator.ts for class-transformer integration
- Update connection.dto.ts with sanitization decorators
- Update identity-linking.dto.ts with sanitization decorators
- Update command.dto.ts with sanitization decorators
- Add comprehensive test coverage including attack vectors

Part of M7.1 Remediation Sprint P1 security fixes.

Fixes #285

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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

@@ -6,6 +6,7 @@
 
 import { IsString, IsObject, IsNotEmpty, IsNumber } from "class-validator";
 import type { CommandMessage } from "../types/message.types";
+import { SanitizeObject } from "../../common/decorators/sanitize.decorator";
 
 /**
  * DTO for sending a command to a remote instance
@@ -21,6 +22,7 @@ export class SendCommandDto {
 
   @IsObject()
   @IsNotEmpty()
+  @SanitizeObject()
   payload!: Record<string, unknown>;
 }
 
@@ -42,6 +44,7 @@ export class IncomingCommandDto implements CommandMessage {
 
   @IsObject()
   @IsNotEmpty()
+  @SanitizeObject()
   payload!: Record<string, unknown>;
 
   @IsNumber()