Rather than passing a pointer to a waitgroup into task.Start(..)
functions, handle the waitgroup.Done() at the callsite.
This makes waitgroup handling local to its definition rather than it
being spread out over multiple files. Tasks now simply execute, and
the policy of waiting on them is handled by the caller.
* Fix error string capitalization
Error strings often follow another string. Hence, they should not be
capitalized, unless referencing a name.
* Uncapitalize more error strings
While here, use %v on the error directly, which makes it easier to wrap
the error later with %w if need be.
* Uncapitalize more error strings
While here, rename Url to URL as a nitpick.
* Make mutating metadata ops mutation
* Implement scene generate screenshot
* Remove fetch policy on metadata mutations
* Port UI changes to v2.5
* Set generated image in database
