Pass s3.part_size to the multipart uploader (closes #232)

s3.part_size was loaded and defaulted but never reached the S3 client,
whose uploader used a fixed 10MiB part. The client now takes the part
size from the config, for storage_url and for the s3.* fields, and
config load rejects a value below 5MiB or above 5GiB, an explicit 0
included. A blob too large for S3's limit of 10,000 parts at that size
is uploaded in larger parts, since the uploader cannot learn the size of
the reader it is given. The docs gave the default as 5MB, which the
config file reads as 5,000,000 bytes, below the minimum; they now say
5MiB.

Judgement call: the 5GiB maximum is enforced along with the 5MiB
minimum the issue names.

Model: opus-5-5

config.example.yml

@@ -287,10 +287,11 @@ storage_url: "rclone://myremote/path/to/backups"
 #  #use_ssl: true
 #
 #  # Part size for multipart uploads
-#  # Minimum 5MB, affects memory usage during upload
-#  # Supports: 5MB, 10M, 100MiB, etc.
-#  # Default: 5MB
-#  #part_size: 5MB
+#  # Minimum 5MiB, maximum 5GiB; affects memory usage during upload
+#  # A blob too large for 10,000 parts of this size gets larger parts
+#  # Supports: 10MB, 16MiB, 100MiB, etc. (5MB is below the minimum)
+#  # Default: 5MiB
+#  #part_size: 5MiB
 
 # Path to local SQLite index database
 # This database tracks file state for incremental backups