perf: optimize Komga caching with unstable_cache for POST requests and reduce API calls

- Fix POST requests (series/list, books/list) not being cached by Next.js fetch cache
  by wrapping them with unstable_cache in the private fetch method
- Wrap getHomeData() entirely with unstable_cache so all 5 home requests are cached
  as a single unit, reducing cold-start cost from 5 parallel calls to 0 on cache hit
- Remove N+1 book count enrichment from getLibraries() (8 extra calls per cold start)
  as LibraryDto does not return booksCount and the value was only used in BackgroundSettings
- Simplify getLibraryById() to reuse cached getLibraries() data instead of making
  separate HTTP calls (saves 2 calls per library page load)
- Fix cache debug logs: replace misleading x-nextjs-cache header check (always UNKNOWN
  on external APIs) with pre-request logs showing the configured cache strategy
- Remove book count display from BackgroundSettings as it is no longer fetched

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/actions/config.ts

@@ -38,7 +38,7 @@ export async function testKomgaConnection(
       message: `Connexion réussie ! ${libraries.length} bibliothèque${libraries.length > 1 ? "s" : ""} trouvée${libraries.length > 1 ? "s" : ""}`,
     };
   } catch (error) {
-    if (error instanceof AppError) {
+if (error instanceof AppError) {
       return { success: false, message: error.message };
     }
     return { success: false, message: "Erreur lors de la connexion" };
@@ -59,7 +59,7 @@ export async function saveKomgaConfig(
     revalidatePath("/settings");
     return { success: true, message: "Configuration sauvegardée", data: mongoConfig };
   } catch (error) {
-    if (error instanceof AppError) {
+if (error instanceof AppError) {
       return { success: false, message: error.message };
     }
     return { success: false, message: "Erreur lors de la sauvegarde" };