The current target length is 99 characters.

Thanks, flake8.

The exceptions to be handled should be specified, otherwise even a
"NameError" would be caught.

Thanks, flake8.

Thanks, flake8.

This solves issues with concurrent Liquidsoap operations.

This temporarly solves:
- engine-core#5

This relates to:
- engine#65
- engine#76

Currently the database session is created during application startup and
is used in all threads.  This is problematic, since the session is not
thread-safe.
Now we use the request-based database session provided by
flask-sqlachemy instead.

But this does not really solve the problem of using the same session in
different threads, since the threads are started within a request.

The introduction of short-lived sessions caused issues with functions,
which return database objects.  Such objects are not accessible (due to
lazy loading), after the session is closed.

Instead we should either move the session scope the respective callers
or use a request-based session handling (e.g. the one provided by
flask-sqlalchemy).

This reverts commit 0b594e50 and 70ffd1a0.

See: #93

Thanks, codespell!

An sqlite database is much easier to setup for simple local deployments
and specifically for test automation.

Previously the database connection ("DB.session") was established during
the startup of Aura Engine.
This was causing conflicts in the threaded execution environment (#75).
Now all sessions are acquired in a short-lived context:

  with DB.Session() as session:
      ...

If database query performance is a real concern, then the session
contexts could be moved to a higher level.

Closes: #75