bingo: store photos in their own bucket

The bingo photos sat under bingo/ in the invoice bucket. Store them in
BINGO_S3_BUCKET (datacenterbingo-photos in prod, created and pre-filled
by the storage repo with the same keys) and fall back to S3_BUCKET when
unset, so dev and tests need nothing new.

Assisted-by: Claude:opus-5

lib/s3.rb

@@ -30,7 +30,7 @@ module S3
     ENV.fetch('S3_BUCKET')
   end
 
-  def put(key, bytes, content_type: 'application/pdf')
+  def put(key, bytes, content_type: 'application/pdf', bucket: self.bucket)
     client.put_object(
       bucket:       bucket,
       key:          key,
@@ -40,7 +40,7 @@ module S3
     key
   end
 
-  def get(key)
+  def get(key, bucket: self.bucket)
     client.get_object(bucket: bucket, key: key).body.read.b # body is a UTF-8 StringIO
   end