refactor(discover): both affinities weigh a watch by one rule

The YouTube channel affinity and the Plex one each had the recency decay, the undated-watch rule and
the scale to the strongest; app.draw now holds them. Tests hold that another user's history leans
neither draw, the rail's blind slot, and that a show leans on show recommendations.

backend/app/plex/affinity.py

@@ -21,6 +21,7 @@ from sqlalchemy.dialects.postgresql import JSONB
 from sqlalchemy.orm import Session
 from sqlalchemy.sql.elements import ColumnElement
 
+from app import draw
 from app.plex.models import PlexItem, PlexShow, PlexState
 from app.userscope import owned_by
 
@@ -28,8 +29,6 @@ HALF_LIFE_DAYS = 180
 RECENT_TITLES = 200
 GENRE_WEIGHT = 2.0
 REC_WEIGHT = 3.0
-# A watch with no date (an old import) counts as one this many half-lives ago.
-_UNDATED_HALF_LIVES = 2
 
 
 @dataclass(frozen=True)
@@ -99,10 +98,7 @@ def for_user(db: Session, uid: int) -> Affinity:
     movie_recs: dict[str, float] = {}
     show_recs: dict[str, float] = {}
     for watched_at, item_genres, item_recs, show_genres, show_recs_json in rows:
-        if watched_at is None:
-            r = 0.5**_UNDATED_HALF_LIVES
-        else:
-            r = 0.5 ** (max(0.0, (now - watched_at).total_seconds() / 86400) / HALF_LIFE_DAYS)
+        r = draw.recency(watched_at, now, HALF_LIFE_DAYS)
         for g in (show_genres if show_genres is not None else item_genres) or []:
             genres[g] = genres.get(g, 0.0) + r
         for rec in (show_recs_json if show_recs_json is not None else item_recs) or []:
@@ -111,10 +107,4 @@ def for_user(db: Session, uid: int) -> Affinity:
             bucket = show_recs if rec.get("media_type") == "tv" else movie_recs
             guid = f"tmdb:{rec['tmdb_id']}"
             bucket[guid] = max(bucket.get(guid, 0.0), r)
-    return Affinity(_scaled(genres), _scaled(movie_recs), _scaled(show_recs))
-
-
-def _scaled(scores: dict[str, float]) -> dict[str, float]:
-    """Each score over the strongest, so the strongest is 1."""
-    top = max(scores.values(), default=0.0)
-    return {k: v / top for k, v in scores.items()} if top else {}
+    return Affinity(draw.scaled(genres), draw.scaled(movie_recs), draw.scaled(show_recs))