Introduce flask-less CLI interface (e.g. for programmatic DB access)

[jotaen4tinypilot]

Resolves #1690.
(Successor of https://github.com/tiny-pilot/tinypilot-pro/pull/1602.)

This PR introduces a new, internal Flask-less CLI entrypoint to our app, for programmatic access of certain features (mainly DB-related).

As a first use-case, it migrates the streaming-mode CLI command, mainly to demonstrate how this is intended to be used. We’ll soon add another command for retrieving the serial settings. In a subsequent step, we can also migrate the last remaining command for activating a license and thereby sunset the Flask CLI entrypoint altogether. (I’d create a separate ticket for that.)

Notes

• Although the footprint of the CLI functionality is still quite small (and is unlikely to grow substantially in the near-to-mid future), I thought that a separate Python module (directory) was warranted (app/cli). That way, we hopefully have a clearer separation from the web app, and we are also a bit more flexible to structure the code nicely.
• I introduced a @command annotation to make it convenient to register commands. My thinking is not to go overboard with this, though – e.g., I’d not expand this mechanism to include arg-parsing etc., as it would then probably make more sense to rather keep the Flask CLI or introduce another framework. The main intention of @command is to have all command-related code closely together, as opposed to having to define the function in one place and then “register” it in a second. Since the whole CLI is more of an internal helper utility, I think it’s fair to keep the whole structure basic.

[jotaen4tinypilot]

Automated comment from CodeApprove ➜

⏳ @jdeanwallace please review this Pull Request

[jdeanwallace]

Automated comment from CodeApprove ➜

✔️ Approved

LGTM!

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👀 @jotaen4tinypilot it's your turn please take a look