Add platform-specific query optimizations

- hackernews: use extract_core_subject instead of raw topic, add
  points>5 filter and restrictSearchableAttributes=title to reduce
  noise from URL-match and low-signal posts
- youtube: add --dateafter parameter to yt-dlp for server-side date
  filtering (Python soft filter still handles fallback)
- reddit: skip opinion/review query variant for how_to/comparison
  queries where it adds noise
- bird_x: add OR-group retry with compound terms before falling back
  to word-dropping (uses X OR operator for multi-concept queries)
- query.py: add detect_query_type() and extract_compound_terms()

scripts/lib/youtube_yt.py

@@ -100,15 +100,16 @@ def search_youtube(
     _log(f"Searching YouTube for '{core_topic}' (since {from_date}, count={count})")
 
     # yt-dlp search with full metadata (no --flat-playlist so dates are real).
-    # No --dateafter — we filter by date in Python with a soft fallback,
-    # because YouTube search returns relevance-sorted results and strict date
-    # filtering returns 0 for evergreen topics like "thumbnail tips".
+    # --dateafter helps yt-dlp filter server-side, but Python soft filter
+    # (below) handles the fallback for evergreen topics with 0 recent results.
+    dateafter = from_date.replace("-", "")  # YYYYMMDD format for yt-dlp
     cmd = [
         "yt-dlp",
         f"ytsearch{count}:{core_topic}",
         "--dump-json",
         "--no-warnings",
         "--no-download",
+        "--dateafter", dateafter,
     ]
 
     preexec = os.setsid if hasattr(os, 'setsid') else None