feat(web): Core Refactor Phase A — extract sampling and cache modules; add adaptive TTL + eviction heuristics, Redis PoC, and metrics wiring. Tests added for TTL, eviction, exports, splash-adaptive, card index, and service worker. Docs+roadmap updated.

code/tests/test_preview_bg_refresh_thread.py

@@ -0,0 +1,23 @@
+import time
+from importlib import reload
+
+from code.web.services import preview_cache as pc
+from code.web.services import theme_preview as tp
+
+
+def test_background_refresh_thread_flag(monkeypatch):
+    # Enable background refresh via env
+    monkeypatch.setenv("THEME_PREVIEW_BG_REFRESH", "1")
+    # Reload preview_cache to re-evaluate env flags
+    reload(pc)
+    # Simulate a couple of builds to trigger ensure_bg_thread
+    # Use a real theme id by invoking preview on first catalog slug
+    from code.web.services.theme_catalog_loader import load_index
+    idx = load_index()
+    slug = sorted(idx.slug_to_entry.keys())[0]
+    for _ in range(2):
+        tp.get_theme_preview(slug, limit=4)
+        time.sleep(0.01)
+    # Background thread flag should be set if enabled
+    assert getattr(pc, "_BG_REFRESH_ENABLED", False) is True
+    assert getattr(pc, "_BG_REFRESH_THREAD_STARTED", False) is True, "background refresh thread did not start"