feat(#293): implement retry logic with exponential backoff

Add retry capability with exponential backoff for HTTP requests.
- Implement withRetry utility with configurable retry logic
- Exponential backoff: 1s, 2s, 4s, 8s (max)
- Maximum 3 retries by default
- Retry on network errors (ECONNREFUSED, ETIMEDOUT, etc.)
- Retry on 5xx server errors and 429 rate limit
- Do NOT retry on 4xx client errors
- Integrate with connection service for HTTP requests

Fixes #293

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

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

@@ -22,6 +22,7 @@ import { firstValueFrom } from "rxjs";
 import type { ConnectionRequest, ConnectionDetails } from "./types/connection.types";
 import type { PublicInstanceIdentity } from "./types/instance.types";
 import { FEDERATION_PROTOCOL_VERSION } from "./constants";
+import { withRetry } from "./utils/retry";
 
 @Injectable()
 export class ConnectionService {
@@ -87,10 +88,19 @@ export class ConnectionService {
     const signature = await this.signatureService.signMessage(request);
     const signedRequest: ConnectionRequest = { ...request, signature };
 
-    // Send connection request to remote instance
+    // Send connection request to remote instance with retry logic
     try {
-      await firstValueFrom(
-        this.httpService.post(`${remoteUrl}/api/v1/federation/incoming/connect`, signedRequest)
+      await withRetry(
+        async () => {
+          return await firstValueFrom(
+            this.httpService.post(`${remoteUrl}/api/v1/federation/incoming/connect`, signedRequest)
+          );
+        },
+        {
+          maxRetries: 3,
+          initialDelay: 1000, // 1s
+          maxDelay: 8000, // 8s
+        }
       );
       this.logger.log(`Connection request sent to ${remoteUrl}`);
     } catch (error) {
@@ -368,13 +378,24 @@ export class ConnectionService {
   }
 
   /**
-   * Fetch remote instance identity via HTTP
+   * Fetch remote instance identity via HTTP with retry logic
    */
   private async fetchRemoteIdentity(remoteUrl: string): Promise<PublicInstanceIdentity> {
     try {
       const normalizedUrl = this.normalizeUrl(remoteUrl);
-      const response = await firstValueFrom(
-        this.httpService.get<PublicInstanceIdentity>(`${normalizedUrl}/api/v1/federation/instance`)
+      const response = await withRetry(
+        async () => {
+          return await firstValueFrom(
+            this.httpService.get<PublicInstanceIdentity>(
+              `${normalizedUrl}/api/v1/federation/instance`
+            )
+          );
+        },
+        {
+          maxRetries: 3,
+          initialDelay: 1000, // 1s
+          maxDelay: 8000, // 8s
+        }
       );
 
       return response.data;