fix cloud library scale and playback compatibility

Root cause: display-only cloud library filtering was reused by scan jobs, merged cloud mounts could be skipped, and OpenList listing relied on WebDAV/first-page behavior that could cap huge directories around 100 items. Cloud scans also let new items consume probe budget before existing rows with missing track/artwork metadata.\n\nChanges:\n- split display filtering from scannable cloud filtering so merged cloud mounts still scan\n- add OpenList API pagination with WebDAV fallback\n- prioritize existing cloud media missing metadata before new imports\n- restrict automatic cloud sync to one successful 19:00-21:00 daily window and keep manual scan immediate\n- disable startup cloud full-scan by default, with an explicit opt-in setting\n- improve Emby/cloud playback compatibility and cache/search/test coverage from the continued work

cmd/server/main_test.go

@@ -72,14 +72,23 @@ func TestServeSPAServesAssetsImmutableAndBypassesAPIRoutes(t *testing.T) {
 		t.Fatalf("asset Cache-Control = %q, want immutable", got)
 	}
 
-	apiReq := httptest.NewRequest(http.MethodGet, "/api/missing", nil)
-	apiResp := httptest.NewRecorder()
-	router.ServeHTTP(apiResp, apiReq)
-	if apiResp.Code != http.StatusNotFound {
-		t.Fatalf("api fallback status = %d, want 404", apiResp.Code)
-	}
-	if strings.Contains(apiResp.Body.String(), "index") {
-		t.Fatalf("api route should not serve SPA index: %q", apiResp.Body.String())
+	for _, path := range []string{
+		"/api/missing",
+		"/emby",
+		"/emby/missing",
+		"/Startup/Configuration",
+		"/QuickConnect/Enabled",
+		"/embywebsocket",
+	} {
+		req := httptest.NewRequest(http.MethodGet, path, nil)
+		resp := httptest.NewRecorder()
+		router.ServeHTTP(resp, req)
+		if resp.Code != http.StatusNotFound {
+			t.Fatalf("%s fallback status = %d, want 404", path, resp.Code)
+		}
+		if strings.Contains(resp.Body.String(), "index") {
+			t.Fatalf("%s should not serve SPA index: %q", path, resp.Body.String())
+		}
 	}
 }