feat(plex): a Surprise reshuffle deals the previous draw last

The rail draws three unseen titles per kind under the sidebar filters; on a
small filtered pool every reshuffle dealt the same few again. The redraw now
sends the draw on screen (exclude=), the server deals the rest of the pool
first and tops up from the excluded titles only when the pool is short, so a
small pool still fills the rail. (The two discover.py baseline entries went
with the rewritten draw.)

backend/app/routes/plex/discover.py

@@ -402,11 +402,31 @@ def discover(
     }
 
 
+def _draw(db: Session, stmt, key_col, per: int, exclude: list[str]) -> list:
+    """`per` random rows of `stmt`, the previous draw's (`exclude`) left out while the pool allows: a
+    reshuffle over a small filtered pool kept dealing the same few titles. What the fresh rows leave
+    short is topped up from the excluded ones, so a pool smaller than a draw still fills the rail."""
+    fresh = (
+        db.execute(stmt.where(key_col.not_in(exclude)).order_by(func.random()).limit(per))
+        .scalars()
+        .all()
+        if exclude
+        else []
+    )
+    if len(fresh) == per:
+        return list(fresh)
+    taken = [key_col.not_in([r.rating_key for r in fresh])] if fresh else []
+    rest = db.execute(stmt.where(*taken).order_by(func.random()).limit(per - len(fresh))).scalars()
+    return [*fresh, *rest.all()]
+
+
 @router.get("/surprise")
 def surprise(
     scope: str = "both",
     lang: str = "en",
     limit: int = Query(default=2 * SURPRISE_PER_KIND, ge=1, le=40),
+    # The previous draw's rating keys: a reshuffle deals other titles first.
+    exclude: list[str] = Query(default=[], max_length=40),
     f: LibraryFilters = Depends(),
     user: User = Depends(current_user),
     db: Session = Depends(get_db),
@@ -432,23 +452,21 @@ def surprise(
             )
             .exists()
         )
-        movies = (
-            db.execute(
-                _movie_meta(
-                    select(PlexItem).where(
-                        PlexItem.kind == "movie",
-                        PlexItem.library_id.in_(movie_ids),
-                        ~watched_movie,
-                    ),
-                    p,
-                    lang,
-                    f,
-                )
-                .order_by(func.random())
-                .limit(per)
-            )
-            .scalars()
-            .all()
+        movies = _draw(
+            db,
+            _movie_meta(
+                select(PlexItem).where(
+                    PlexItem.kind == "movie",
+                    PlexItem.library_id.in_(movie_ids),
+                    ~watched_movie,
+                ),
+                p,
+                lang,
+                f,
+            ),
+            PlexItem.rating_key,
+            per,
+            exclude,
         )
     shows = []
     if show_ids:
@@ -458,21 +476,19 @@ def surprise(
         not_fully_watched = or_(
             prog.c.total.is_(None), prog.c.total == 0, prog.c.watched < prog.c.total
         )
-        shows = (
-            db.execute(
-                apply_meta_filters(
-                    select(PlexShow)
-                    .outerjoin(prog, prog.c.show_id == PlexShow.id)
-                    .where(PlexShow.library_id.in_(show_ids), not_fully_watched),
-                    PlexShow,
-                    p,
-                    lang,
-                )
-                .order_by(func.random())
-                .limit(per)
-            )
-            .scalars()
-            .all()
+        shows = _draw(
+            db,
+            apply_meta_filters(
+                select(PlexShow)
+                .outerjoin(prog, prog.c.show_id == PlexShow.id)
+                .where(PlexShow.library_id.in_(show_ids), not_fully_watched),
+                PlexShow,
+                p,
+                lang,
+            ),
+            PlexShow.rating_key,
+            per,
+            exclude,
         )
     mcards = _movie_cards(db, user.id, movies, lang)
     scards = _show_cards(db, user.id, shows, lang)