When logged in, we automatically save a program as the user runs it.

In a recent change, we update the program counter only when a program is
modified from the example code. But we wrote the entire 'program' record
back to the database in an `update()` call, which includes the `id`
field; Dynamo does not allow including the `id` field in an `update()`
call.

Instead, only update the `is_modified` field, and update the Dynamo
abstraction layer to check for this error condition.

for more information, see https://pre-commit.ci

for more information, see https://pre-commit.ci

…y-update