move config parsing to own module

[brassy-endomorph]

fixes #637

This is a minimal change to organize the config parsing (somewhat) according to some logic that seemed reasonable to me (for now). There are some slight change in the parsing logic so we don't set empty/None values in a bunch of places since this make some logic a little cleaner so we can do tests like if "KEY" in app.config which is different than if app.config.get("KEY").

• moves all config parsing to config.py
• parses random env vars prefixed with HL_CFG_
• parses randm env vars prefixed with HL_CFG_JSON_ as JSON
• basic tests for parsing
• updated docs
• bumped dependencies because somehow that fixed linting (???)

[glenn-sorrentino]

Loggin in causes this:

    File "/root/.cache/pypoetry/virtualenvs/hushline-9TtSrW0h-py3.12/lib/python3.12/site-packages/flask/app.py", line 1498, in __call__

    return self.wsgi_app(environ, start_response)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    File "/root/.cache/pypoetry/virtualenvs/hushline-9TtSrW0h-py3.12/lib/python3.12/site-packages/flask/app.py", line 1476, in wsgi_app

    response = self.handle_exception(e)
               ^^^^^^^^^^^^^^^^^^^^^^^^

    File "/root/.cache/pypoetry/virtualenvs/hushline-9TtSrW0h-py3.12/lib/python3.12/site-packages/flask/app.py", line 1473, in wsgi_app

    response = self.full_dispatch_request()
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    File "/root/.cache/pypoetry/virtualenvs/hushline-9TtSrW0h-py3.12/lib/python3.12/site-packages/flask/app.py", line 882, in full_dispatch_request

    rv = self.handle_user_exception(e)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    File "/root/.cache/pypoetry/virtualenvs/hushline-9TtSrW0h-py3.12/lib/python3.12/site-packages/flask/app.py", line 880, in full_dispatch_request

    rv = self.dispatch_request()
         ^^^^^^^^^^^^^^^^^^^^^^^

    File "/root/.cache/pypoetry/virtualenvs/hushline-9TtSrW0h-py3.12/lib/python3.12/site-packages/flask/app.py", line 865, in dispatch_request

    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)  # type: ignore[no-any-return]
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    File "/app/hushline/routes.py", line 412, in login

    if app.config["STRIPE_SECRET_KEY"]:
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    KeyError: 'STRIPE_SECRET_KEY'

[brassy-endomorph]

I didn't actually test it yet. Just got it to pass CI, and this check is whether this method looks sensible. But yes, noted.