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/signature.service.ts

@@ -156,23 +156,22 @@ export class SignatureService {
    * @returns A new object with sorted keys
    */
   private sortObjectKeys(obj: SignableMessage): SignableMessage {
-    // Handle null
-    if (obj === null) {
-      return obj;
-    }
-
-    // Handle arrays - map recursively
+    // Handle null and primitives
     // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
-    if (Array.isArray(obj)) {
-      // eslint-disable-next-line @typescript-eslint/no-explicit-any, @typescript-eslint/no-unsafe-return, @typescript-eslint/no-unsafe-argument
-      return obj.map((item: any) =>
-        typeof item === "object" && item !== null ? this.sortObjectKeys(item) : item
-      ) as SignableMessage;
+    if (obj === null || typeof obj !== "object") {
+      return obj;
     }
 
-    // Handle non-objects (primitives)
-    if (typeof obj !== "object") {
-      return obj;
+    // Handle arrays - recursively sort elements
+    if (Array.isArray(obj)) {
+      const sortedArray = obj.map((item: unknown) => {
+        if (typeof item === "object" && item !== null) {
+          return this.sortObjectKeys(item as SignableMessage);
+        }
+        return item;
+      });
+      // Arrays are valid SignableMessage values when nested in objects
+      return sortedArray as unknown as SignableMessage;
     }
 
     // Handle objects - sort keys alphabetically
@@ -181,10 +180,11 @@ export class SignatureService {
 
     for (const key of keys) {
       const value = obj[key];
-      sorted[key] =
-        typeof value === "object" && value !== null
-          ? this.sortObjectKeys(value as SignableMessage)
-          : value;
+      if (typeof value === "object" && value !== null) {
+        sorted[key] = this.sortObjectKeys(value as SignableMessage);
+      } else {
+        sorted[key] = value;
+      }
     }
 
     return sorted;