feat: add multi-provider support (Komga + Stripstream Librarian)

- Introduce provider abstraction layer (IMediaProvider, KomgaProvider, StripstreamProvider)
- Add Stripstream Librarian as second media provider with full feature parity
- Migrate all pages and components from direct Komga services to provider factory
- Remove dead service code (BaseApiService, HomeService, LibraryService, SearchService, TestService)
- Fix library/series page-based pagination for both providers (Komga 0-indexed, Stripstream 1-indexed)
- Fix unread filter and search on library page for both providers
- Fix read progress display for Stripstream (reading_status mapping)
- Fix series read status (books_read_count) for Stripstream
- Add global search with series results for Stripstream (series_hits from Meilisearch)
- Fix thumbnail proxy to return 404 gracefully instead of JSON on upstream error
- Replace duration-based cache debug detection with x-nextjs-cache header

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

src/lib/providers/provider.factory.ts

@@ -0,0 +1,52 @@
+import prisma from "@/lib/prisma";
+import { getCurrentUser } from "@/lib/auth-utils";
+import type { IMediaProvider } from "./provider.interface";
+
+export async function getProvider(): Promise<IMediaProvider | null> {
+  const user = await getCurrentUser();
+  if (!user) return null;
+
+  const userId = parseInt(user.id, 10);
+
+  const dbUser = await prisma.user.findUnique({
+    where: { id: userId },
+    select: {
+      activeProvider: true,
+      config: { select: { url: true, authHeader: true } },
+      stripstreamConfig: { select: { url: true, token: true } },
+    },
+  });
+
+  if (!dbUser) return null;
+
+  const activeProvider = dbUser.activeProvider ?? "komga";
+
+  if (activeProvider === "stripstream" && dbUser.stripstreamConfig) {
+    const { StripstreamProvider } = await import("./stripstream/stripstream.provider");
+    return new StripstreamProvider(
+      dbUser.stripstreamConfig.url,
+      dbUser.stripstreamConfig.token
+    );
+  }
+
+  if (activeProvider === "komga" || !dbUser.activeProvider) {
+    if (!dbUser.config) return null;
+    const { KomgaProvider } = await import("./komga/komga.provider");
+    return new KomgaProvider(dbUser.config.url, dbUser.config.authHeader);
+  }
+
+  return null;
+}
+
+export async function getActiveProviderType(): Promise<string | null> {
+  const user = await getCurrentUser();
+  if (!user) return null;
+
+  const userId = parseInt(user.id, 10);
+  const dbUser = await prisma.user.findUnique({
+    where: { id: userId },
+    select: { activeProvider: true },
+  });
+
+  return dbUser?.activeProvider ?? "komga";
+}