fix(feed): an out-of-range seed or cursor is refused, not a 500

A seed past a float overflowed the shuffle's start point, and a cursor holding Infinity or a 400-digit
key overflowed its int/float read: both answered 500. The routes bound the seed (422), the start
point folds it to 32 bits, and the cursor read counts an overflow as a bad cursor (400). A plan test
holds that the walk reads ix_videos_shuffle_key with no sort over a realistic scope.

backend/app/feedquery.py

@@ -719,8 +719,11 @@ _GOLDEN = 0.6180339887498949
 
 
 def shuffle_start(seed: int) -> float:
-    """The point in [0, 1) where a seed's shuffle starts on the ring of `Video.shuffle_key`."""
-    return (seed * _GOLDEN) % 1.0
+    """The point in [0, 1) where a seed's shuffle starts on the ring of `Video.shuffle_key`.
+
+    Folded to 32 bits first: an int past a float's range overflowed the product, and one past 2**53
+    left the product no fraction to land on. A seed the client draws (under a billion) is unchanged."""
+    return ((seed % 2**32) * _GOLDEN) % 1.0
 
 
 def shuffle_walk(
@@ -787,7 +790,7 @@ def feed_page(
         if vals is not None and cursor_id is not None:
             try:
                 after = (int(vals[0]), float(vals[1]), str(cursor_id))
-            except (TypeError, ValueError) as e:
+            except (TypeError, ValueError, OverflowError) as e:  # Infinity, a 400-digit key
                 raise BadCursor("Invalid feed cursor") from e
         rows = shuffle_walk(db, query, seed, limit + 1, after)
     else: