Enricher: don't spawn the background daemon under pytest (test isolation)

The enrichment daemon was starting during API tests (it uses the default DB +
network), touching a stray real DB and causing sqlite disk-IO flakiness. enqueue()
now no-ops when PYTEST_CURRENT_TEST is set; _enrich() is still tested directly
with a tmp DB. 176 video tests green.

core/video/youtube_enrichment.py

@@ -13,6 +13,7 @@ followed (or its page opened while followed). Reads/writes only video_library.db
 
 from __future__ import annotations
 
+import os
 import queue
 import threading
 import time
@@ -50,6 +51,10 @@ class YoutubeDateEnricher:
         cid = str(channel_id or "").strip()
         if not cid:
             return
+        # Never spawn the background daemon (network + the default DB) under tests;
+        # the enricher's logic is exercised directly via _enrich() with a tmp DB.
+        if os.environ.get("PYTEST_CURRENT_TEST"):
+            return
         with self._lock:
             if title:
                 self._titles[cid] = title