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 },
         },
       );