fix: live YouTube search 500 on the scrape source (published_at NOT NULL)

The scrape source exposes no publish date, so its search stub carried
published_at=None. Migration 0068 (PERF-15, E2/0.68.0) made
videos.published_at NOT NULL, so insert_stubs raised IntegrityError —
outside the route's page-fetch try/except — and the whole
/api/search/youtube request 500'd ("YouTube search failed."). Every
scrape search (the default source) had been broken since 0.68.0.

- routes/search._ingest_candidates: stub published_at falls back to
  now() (mirrors stub_from_playlist_item), satisfying the constraint.
- sync.videos.apply_video_details: prefer the enrich snippet's real
  date whenever present, so the now() placeholder is corrected instead
  of becoming the video's permanent date (the guard only filled a NULL,
  which a non-null placeholder no longer is). Never clobbers a real
  date with now().

Regression tests (DB lane): dateless scrape stub ingests and takes the
real enrich date; an enrich miss keeps a non-null placeholder, unkept.

backend/app/sync/videos.py

@@ -245,9 +245,15 @@ def apply_video_details(video: Video, item: dict) -> None:
         video.original_title = raw_title
         video.title = normalize_title(raw_title)
     video.description = snippet.get("description")
-    if not video.published_at:
-        # NOT NULL (0068): a fresh insert with no date yet falls back to now() (enrich normally has one).
-        video.published_at = parse_dt(snippet.get("publishedAt")) or _now()
+    # published_at is NOT NULL (0068). Prefer the enrich snippet's real date whenever it has one —
+    # this also corrects a now() placeholder written at stub time (e.g. a scrape-search stub, which
+    # carries no publish date). Only fall back to now() when the row still has nothing at all; never
+    # clobber an existing real date with now() (a re-enrich whose snippet lacks a date).
+    snippet_published = parse_dt(snippet.get("publishedAt"))
+    if snippet_published:
+        video.published_at = snippet_published
+    elif not video.published_at:
+        video.published_at = _now()
     video.thumbnail_url = best_thumbnail(snippet.get("thumbnails")) or video.thumbnail_url
     video.duration_seconds = parse_iso8601_duration(content.get("duration"))
     video.view_count = int(stats["viewCount"]) if stats.get("viewCount") else None