Migrate backend from Flask to FastAPI

[mhauru]

Changes in this PR:

• Change the backend from Flask to FastAPI
• Change the input/output of a couple of endpoints. I only did this in cases where I considered the old one to be essentially a bug.
• Change many endpoints from GET or DELETE to POST. This is because in the process of switching to FastAPI I learned that GET and DELETE endpoints should never require JSON bodies in the requests. If a JSON body is required, the method should be POST (or PUT). I wonder if we should just change all of the endpoints to just use POST, for simplicity?
• Various small bugfixes, better type annotations, etc. that came up on the way.

Benefits of this switch:

• The backend should be much faster now. Hasn't been a problem so far, but may be useful later.
• Much less boiler plate code for validating inputs into the endpoints. Previously we were manually checking that the JSON in the response had the right keys etc. Now we define a Pydantic model class that sets the keys and their expected types, and FastAPI/Pydantic do the validation and parsing automatically. Same for outgoing responses.
• Much more robust and comprehensive input validation. See above.
• Automated backend documentation. No more capturing the inputs/outputs of the endpoints in three places: The code, the docstring, and the backend README file. FastAPI autogenerates docs from the code. If you're running the backend locally, navigate to localhost:5000/docs to see the documentation. This was the main reason why I initiated the switch.

Downsides of this switch:

• We needed to implement more of the backend authentication logic manually, since we can't use the Flask plugin for this anymore. This is all in dtbase/backend/auth.py.
• We needed to implement more of the database connection management logic manually, since we can't use the Flask plugin for this anymore. I actually might consider this neutral/positive though, because the way the Flask did this, which relied heavily on some pseudoglobal variables that had different values depending on which context you were reading them, was a bit magical and confusing. The new code for this is in dtbase/backend/db.py.

Things not done in this PR, but that we may want to do in the future. None of these are urgent though, so not going to get to them immediately:

• Restructuring the directory structure of the backend. See #129
• Improve the documentation of API endpoint outputs. I think we could probably get the automated docs to have more detail about the meaning of the various fields by adding some arguments to pydantic.Field objects. The inputs are already quite comprehensively documented.
• Switch all endpoints to use POST as the method. Not sure about this one, but I'm bothered by how some endpoints are GET and others POST even though their functions are very similar. See #210
• Use the same Pydantic models that specify the return types of endpoints to add type annotations to the frontend and tests. In other words, make use of the fact that we know exactly what the return types of our endpoints are now. #209

Closes #167

[mhauru]

The tests fail because of some datetime parsing issue, I'm pretty sure they pass on python 3.11 (they do locally). I'll fix it tomorrow.

[EdwinB12]

Interesting, I had this yesterday. Test fails on one of them and then cancels it on python runs. Rerunning the tests through the github gui seems to work

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.05628% with 34 lines in your changes are missing coverage. Please review.

Project coverage is 89.65%. Comparing base (38cbdc6) to head (ba20199).

:exclamation: Current head ba20199 differs from pull request most recent head c1cc874. Consider uploading reports for the commit c1cc874 to get more accurate results

Files	Patch %	Lines
dtbase/backend/auth.py	84.78%	7 Missing :warning:
dtbase/backend/api/sensor/routes.py	95.28%	5 Missing :warning:
dtbase/backend/db.py	83.33%	4 Missing :warning:
dtbase/tests/conftest.py	91.66%	4 Missing :warning:
dtbase/tests/test_api_models.py	98.13%	3 Missing :warning:
dtbase/backend/api/location/routes.py	97.61%	2 Missing :warning:
dtbase/backend/api/model/routes.py	98.23%	2 Missing :warning:
dtbase/tests/test_api_sensors.py	98.29%	2 Missing :warning:
dtbase/tests/test_api_user.py	96.22%	2 Missing :warning:
dtbase/core/db.py	87.50%	1 Missing :warning:
... and 2 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #207 +/- ## ========================================== + Coverage 89.44% 89.65% +0.21% ========================================== Files 76 72 -4 Lines 5287 5116 -171 ========================================== - Hits 4729 4587 -142 + Misses 558 529 -29 ```

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[mhauru]

Thanks for the speedy review Edwin. I'll merge once tests pass. @GiorgioCerro, feel free to still leave comments if you want to, we can always make post-merge changes.