feat(titles): normalize video titles for display (feed, search, downloads)

Noisy YouTube titles are cleaned for display + storage, raw kept in videos.original_title:
- app/titles.normalize_title: strip emoji/symbols (keep accents), remove trailing SEO hashtag
  clusters (keep numeric #3 episode markers), context-aware de-shout (mostly-ALL-CAPS titles ->
  Title Case with an acronym whitelist + function-word lowercasing; otherwise only long all-caps
  words), collapse repeated punctuation
- applied at enrichment (sync/videos.py) and in the download worker (ad-hoc yt-dlp titles);
  catalog downloads inherit the normalized title automatically
- migration 0039: add original_title, preserve raw, rewrite title (generated search_vector
  regenerates); reversible via original_title

Backfill on localdev: 122115/273417 titles normalized in ~2 min. Verified in the feed + on
real messy samples (emoji/de-shout/hashtags), accents + acronyms (PS5/AI/USA/PC) preserved.

backend/app/models.py

@@ -241,7 +241,8 @@ class Video(Base, TimestampMixin):
     channel_id: Mapped[str] = mapped_column(
         ForeignKey("channels.id", ondelete="CASCADE"), index=True
     )
-    title: Mapped[str | None] = mapped_column(Text)
+    title: Mapped[str | None] = mapped_column(Text)  # normalized for display (see app.titles)
+    original_title: Mapped[str | None] = mapped_column(Text)  # raw YouTube title, preserved
     description: Mapped[str | None] = mapped_column(Text)
     published_at: Mapped[datetime | None] = mapped_column(
         DateTime(timezone=True), index=True