CRINGE-206: Migrate from pip and pip-tools to uv.

[smarnach]

This change migrates Tecken to uv. See https://hackmd.io/@smarnach/rkzaaZFPZx for more details.

This also includes some tangentially related clean-ups:

• The .dockerignore file was out of data. It included references to things that no longer exist, and was missing a few entries it should have.
• When building the frontend container, we only copy the files we actually need to build the frontend. This makes Docker caching effective; otherwise any change to any file in the entire directory would trigger a rebuild from scratch.
• The Sentry scrub test includes hardcoded paths to Python files. These used to change on every Python version upgrade, and they changed with this migration as well since they are now pointing into uv's managed venv. I made the code a bit more generic in the hope we no longer need to keep updating these paths.

https://mozilla-hub.atlassian.net/browse/CRINGE-206