refactor: make PALDRA match PDCA style exactly - remove redundant instanceConfig parameter and convert to pure singleton

- Remove instanceConfig parameter from all retriever methods (getTags, getQueue, getHistory)
- Retriever instances now use this.url, this.apiKey, this.id instead of passed parameter
- Convert ArrRetrieverRegistry from class with convenience functions to pure singleton object
- Export singleton instance directly instead of class + convenience functions
- Update poller.js and historyFetcher.js to call methods on singleton directly
- All 261 tests pass with zero behavior changes

server/utils/historyFetcher.js

@@ -1,7 +1,7 @@
 // Copyright (c) 2026 Gordon Bolton. MIT License.
 const cache = require('./cache');
 const { getSonarrInstances, getRadarrInstances } = require('./config');
-const { initializeRetrievers, getRetrieversByType } = require('./arrRetrievers');
+const arrRetrieverRegistry = require('./arrRetrievers');
 
 // Cache TTL for recent-history data: 5 minutes.
 // History changes slowly compared to active downloads.
@@ -27,17 +27,17 @@ async function fetchSonarrHistory(since) {
   if (cached) return cached;
 
   // Ensure retrievers are initialized
-  await initializeRetrievers();
+  await arrRetrieverRegistry.initialize();
 
   const instances = getSonarrInstances();
-  const sonarrRetrievers = getRetrieversByType('sonarr');
+  const sonarrRetrievers = arrRetrieverRegistry.getRetrieversByType('sonarr');
 
   const results = await Promise.all(sonarrRetrievers.map(async (retriever) => {
     const inst = instances.find(i => i.id === retriever.getInstanceId());
     if (!inst) return [];
 
     try {
-      const response = await retriever.getHistory(retriever, {
+      const response = await retriever.getHistory({
         pageSize: 100,
         sortKey: 'date',
         sortDir: 'descending',
@@ -76,17 +76,17 @@ async function fetchRadarrHistory(since) {
   if (cached) return cached;
 
   // Ensure retrievers are initialized
-  await initializeRetrievers();
+  await arrRetrieverRegistry.initialize();
 
   const instances = getRadarrInstances();
-  const radarrRetrievers = getRetrieversByType('radarr');
+  const radarrRetrievers = arrRetrieverRegistry.getRetrieversByType('radarr');
 
   const results = await Promise.all(radarrRetrievers.map(async (retriever) => {
     const inst = instances.find(i => i.id === retriever.getInstanceId());
     if (!inst) return [];
 
     try {
-      const response = await retriever.getHistory(retriever, {
+      const response = await retriever.getHistory({
         pageSize: 100,
         sortKey: 'date',
         sortDir: 'descending',