Refactor indexer management and improve search logic

Refactors admin settings to use a new IndexersTab and card-based indexer management UI, supporting category selection and improved configuration. Updates backend and API routes to handle indexer categories, propagate ASIN for better search scoring, and group indexers by categories to optimize Prowlarr searches. Enhances documentation to clarify non-terminal request matching and auto-completion behavior. Adds new reusable components for indexer management and category selection.

src/lib/utils/indexer-grouping.ts

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+/**
+ * Utility: Indexer Grouping by Categories
+ * Documentation: documentation/phase3/prowlarr.md
+ *
+ * Groups indexers by their category configuration to minimize API calls.
+ * Indexers with identical categories are grouped together for a single search.
+ */
+
+export interface IndexerConfig {
+  id: number;
+  name: string;
+  priority?: number;
+  categories?: number[];
+  [key: string]: any; // Allow other properties
+}
+
+export interface IndexerGroup {
+  categories: number[];
+  indexerIds: number[];
+  indexers: IndexerConfig[];
+}
+
+/**
+ * Groups indexers by their category configuration.
+ * Indexers with identical category arrays are grouped together.
+ *
+ * @param indexers - Array of indexer configurations
+ * @returns Array of groups, each containing indexers with matching categories
+ *
+ * @example
+ * const indexers = [
+ *   { id: 1, categories: [3030] },
+ *   { id: 2, categories: [3030] },
+ *   { id: 3, categories: [3030, 3010] },
+ * ];
+ *
+ * const groups = groupIndexersByCategories(indexers);
+ * // Result:
+ * // [
+ * //   { categories: [3030], indexerIds: [1, 2], indexers: [...] },
+ * //   { categories: [3030, 3010], indexerIds: [3], indexers: [...] }
+ * // ]
+ */
+export function groupIndexersByCategories(indexers: IndexerConfig[]): IndexerGroup[] {
+  // Map to track unique category combinations
+  // Key: sorted category IDs as string (e.g., "3030,3010")
+  // Value: array of indexers with those categories
+  const groupMap = new Map<string, IndexerConfig[]>();
+
+  for (const indexer of indexers) {
+    // Get categories, default to [3030] (audiobooks) if not specified
+    const categories = indexer.categories && indexer.categories.length > 0
+      ? indexer.categories
+      : [3030];
+
+    // Sort categories to ensure consistent grouping
+    // [3030, 3010] and [3010, 3030] should be the same group
+    const sortedCategories = [...categories].sort((a, b) => a - b);
+    const key = sortedCategories.join(',');
+
+    // Add indexer to group
+    if (!groupMap.has(key)) {
+      groupMap.set(key, []);
+    }
+    groupMap.get(key)!.push(indexer);
+  }
+
+  // Convert map to array of groups
+  const groups: IndexerGroup[] = [];
+  for (const [key, indexersInGroup] of groupMap.entries()) {
+    const categories = key.split(',').map(Number);
+    const indexerIds = indexersInGroup.map(idx => idx.id);
+
+    groups.push({
+      categories,
+      indexerIds,
+      indexers: indexersInGroup,
+    });
+  }
+
+  return groups;
+}
+
+/**
+ * Get a human-readable description of an indexer group.
+ * Useful for logging and debugging.
+ *
+ * @param group - The indexer group
+ * @returns Description string
+ *
+ * @example
+ * const description = getGroupDescription(group);
+ * // "3 indexers (IDs: 1, 2, 5) searching categories [3030, 3010]"
+ */
+export function getGroupDescription(group: IndexerGroup): string {
+  const indexerCount = group.indexerIds.length;
+  const indexerNames = group.indexers.map(idx => idx.name).join(', ');
+  const categoriesStr = group.categories.join(', ');
+
+  return `${indexerCount} indexer${indexerCount > 1 ? 's' : ''} (${indexerNames}) with categories [${categoriesStr}]`;
+}