Add langdetect language filter to enricher + purge non-English ZIM articles

- Install langdetect package for content-level language detection
- Add _check_language() to enricher.py: reads first 1500 chars of first
  page, detects language via langdetect, skips if not in allowed list
- Configurable via config.yaml pipeline.language_filter and
  pipeline.allowed_languages (default: en only)
- Catches non-English content from ANY source (PDF, web, ZIM, PeerTube)
  before burning Gemini API quota on enrichment
- Add scan_zims retry logic (3 attempts, 2s delay) for upload handler
- Purged 6,483 stale non-English zim_articles rows from DB

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

lib/api.py

@@ -2030,12 +2030,16 @@ def api_kiwix_upload():
     except Exception as e:
         logger.warning(f"kiwix-manage add failed: {e}")
 
-    # Scan for new entry
-    try:
-        from .zim_monitor import scan_zims
-        scan_zims()
-    except Exception as e:
-        logger.warning(f"scan_zims after upload failed: {e}")
+    # Scan for new entry (retry — monitorLibrary may need a moment to reload)
+    import time as _time
+    from .zim_monitor import scan_zims
+    for attempt in range(3):
+        try:
+            scan_zims()
+            break
+        except Exception as e:
+            logger.warning(f"scan_zims attempt {attempt+1} failed: {e}")
+            _time.sleep(2)
 
     # Refresh cache
     try: