Add custom search terms & retry download (admin)

Add support for per-request custom search terms and an admin retry-download flow.

- DB/schema: add custom_search_terms column via Prisma migration and schema update.
- Admin UI: new AdjustSearchTermsModal component and UI badges to show custom search status; RequestActionsDropdown and RecentRequestsTable updated to surface adjust/retry actions.
- API: new PATCH /api/admin/requests/[id]/search-terms to set/clear custom terms (optionally trigger a new search) and new POST /api/admin/requests/[id]/retry-download to resume monitoring or re-add downloads using DownloadHistory metadata.
- Behavior: interactive search now prefers customSearchTerms when present; manual import exposes cleanupSource option to organize job; admin requests listing returns downloadAttempts and customSearchTerms.
- UX: add SectionToolbar, LoadMoreBar and HideAvailableToggle components and wire hide-available preference across home, search, author and series pages; authors/series endpoints/page handlers gain pagination metadata.
- Misc: add connection-errors util and update related processors/services and tests to cover the new flows.

These changes enable admins to override search terms per request, trigger searches from the admin UI, and retry failed downloads more robustly.

src/lib/utils/connection-errors.ts

@@ -0,0 +1,80 @@
+/**
+ * Component: Connection Error Classification Utility
+ * Documentation: documentation/phase3/README.md
+ *
+ * Classifies errors as transient connection failures (e.g. download client
+ * restarting, network blip) vs permanent failures.  Used by download
+ * processors to decide whether to retry with backoff or fail immediately.
+ */
+
+/** Node/Axios error codes that indicate the remote service is temporarily unreachable. */
+const TRANSIENT_ERROR_CODES = new Set([
+  'ECONNREFUSED',
+  'ECONNRESET',
+  'ECONNABORTED',
+  'ETIMEDOUT',
+  'ENOTFOUND',
+  'EHOSTUNREACH',
+  'ENETUNREACH',
+  'EPIPE',
+  'EAI_AGAIN',
+]);
+
+/** HTTP status codes that indicate a gateway / upstream service issue. */
+const TRANSIENT_HTTP_STATUSES = new Set([502, 503, 504]);
+
+/**
+ * Substrings in error messages that strongly indicate a connection-level
+ * failure.  Checked as a fallback when structured error properties are
+ * unavailable (e.g. errors re-thrown as plain Error with a message string).
+ */
+const TRANSIENT_MESSAGE_PATTERNS = [
+  'ECONNREFUSED',
+  'ECONNRESET',
+  'ECONNABORTED',
+  'ETIMEDOUT',
+  'ENOTFOUND',
+  'EHOSTUNREACH',
+  'ENETUNREACH',
+  'EPIPE',
+  'EAI_AGAIN',
+  'connect ECONNREFUSED',
+  'socket hang up',
+  'network error',
+  'Client network socket disconnected',
+] as const;
+
+/**
+ * Returns `true` when the error looks like a transient connection failure
+ * rather than a permanent / logical error.
+ *
+ * Checks (in order):
+ *  1. `error.code`  — Node.js / Axios error codes
+ *  2. `error.response.status` — HTTP gateway errors (502/503/504)
+ *  3. `error.message` — fallback substring matching
+ */
+export function isTransientConnectionError(error: unknown): boolean {
+  if (!error) return false;
+
+  // 1. Structured error code (Node.js / Axios)
+  const code = (error as any)?.code;
+  if (typeof code === 'string' && TRANSIENT_ERROR_CODES.has(code)) {
+    return true;
+  }
+
+  // 2. HTTP gateway status from Axios response
+  const status = (error as any)?.response?.status;
+  if (typeof status === 'number' && TRANSIENT_HTTP_STATUSES.has(status)) {
+    return true;
+  }
+
+  // 3. Fallback: substring match on the error message
+  const message = (error instanceof Error ? error.message : String(error)).toUpperCase();
+  for (const pattern of TRANSIENT_MESSAGE_PATTERNS) {
+    if (message.includes(pattern.toUpperCase())) {
+      return true;
+    }
+  }
+
+  return false;
+}