api/schedule: yield a more informative exception if no operation is found

test/api: exhibit issue if we prepare twice the same thing

api/prepare: make sure we pass correctly the prepared input to the actual task

While doing so we tidy the `cleanup` fixture and use it everywhere, to
kill inter-tests interferences.

api/prepare: handle the input spec protocol

monitor: style

doc: indicate the namespace name

doc: remove reference to sqlalchemy

doc: fix titles

doc: update the repository urls

doc/task: document the `domain` and `timeout` parameters

doc/schedule: fix forgotten domain parameter

doc/inputs: show the task.input value

doc/inputs: add datetime example

inputs/datetime: be more robust with dates at pack time

inputs: add datetime handling

inputs: more robust surrogate input handling

doc: something for the input handling

inputs: compress input data with zstd

tests/inputs: make sure it works with an empty input and refuses invalid input

inputs: automatic marshalling at `schedule` time of the speced inputs