Centralize and standardize User-Agent string

Introduce a centralized RMAB_USER_AGENT constant (ReadMeABook/<version>) and update audible service calls to use it instead of hardcoded values. This avoids the default axios UA (which some indexers reject) and replaces the previous `rmab/` identifier. Adds unit tests to verify the User-Agent format and ensure it doesn't resemble generic bot signatures.

src/lib/integrations/audible.service.ts

@@ -677,7 +677,7 @@ export class AudibleService {
         {
           params: { region: audnexusRegion },
           timeout: 10000,
-          headers: { 'User-Agent': 'ReadMeABook/1.0' },
+          headers: { 'User-Agent': RMAB_USER_AGENT },
         },
       );
 
@@ -768,7 +768,7 @@ export class AudibleService {
         {
           params: { region: audnexusRegion },
           timeout: 5000,
-          headers: { 'User-Agent': 'ReadMeABook/1.0' },
+          headers: { 'User-Agent': RMAB_USER_AGENT },
         },
       );
 

src/lib/utils/user-agent.ts

@@ -1,3 +1,11 @@
+/**
+ * Component: User-Agent Constant
+ *
+ * Centralized User-Agent string used for all outbound HTTP requests.
+ * Replaces the default `axios/x.y.z` UA, which is rejected by some
+ * indexers (e.g., NZBFinder) as a generic-bot signature.
+ */
+
 import { version } from '../../../package.json';
 
-export const RMAB_USER_AGENT = `rmab/${version}`;
+export const RMAB_USER_AGENT = `ReadMeABook/${version}`;

tests/lib/utils/user-agent.test.ts

@@ -0,0 +1,13 @@
+import { describe, it, expect } from 'vitest';
+import { RMAB_USER_AGENT } from '@/lib/utils/user-agent';
+
+describe('RMAB_USER_AGENT', () => {
+  it('uses the ReadMeABook/<semver> format', () => {
+    expect(RMAB_USER_AGENT).toMatch(/^ReadMeABook\/\d+\.\d+\.\d+/);
+  });
+
+  it('does not look like a generic bot signature', () => {
+    expect(RMAB_USER_AGENT).not.toMatch(/^axios\//);
+    expect(RMAB_USER_AGENT).not.toMatch(/^rmab\//);
+  });
+});