Add isort config and sort imports

[john-s-morgan]

What this is

Adds isort, a tool that keeps import statements in a consistent order; plus a one-time pass applying it across the codebase.

Why bother

Right now import order is freeform, so it drifts. In practice that costs us three small-but-recurring things:

• Review noise: PRs that add an import often reshuffle nearby lines, and reviewers can't tell the real change from the churn. I hit this while cleaning up Refactor OAuth views and remove PKCE workaround for LiveSplit #231, which is what prompted this PR.
• Merge conflicts: two branches adding imports to the same file collide for no good reason.
• Bikeshedding: a tool answers where the import lives in the file, so nobody has to think about it.

None of these are on fire. This is a papercut fix.

What it does not change

• Zero functional changes. It only reorders/wraps import lines. Every changed file still compiles and behaves identically.
• It matches the style already dominant in the repo (single-line imports, wrapping long ones vertically). I didn't impose a new aesthetic. I measured what the codebase already does and configured isort to preserve it. I have no strong opinions about any of the settings, and am happy to set them to whatever you'd prefer if different.
• Generated migrations are skipped, so Django's output is left alone.
• racetime/views/__init__.py is skipped: its imports are deliberately hand-ordered to pair each view with its OAuth variant (RaceChatPin next to OAuthRaceChatPin). The config respects that instead of flattening it, since we can't auto-sort while preserving that intent.

How it's structured

Two commits, so you can review the decision separately from the churn:

1. Add isort configuration: just .isort.cfg (~9 lines). This is the only part that's a decision; read this one.
2. Sort imports with isort: the mechanical result of running the tool. 27 files, all reordering. You can skim or trust it; it's reproducible by running isort ..

What you have to do going forward

Nothing required. If you ever want to keep it tidy, pip install isort && isort . reformats everything; running it twice is a no-op. If you decide you hate it, deleting .isort.cfg reverts to freeform with no other cleanup.

If you love it and want it enforced automatically

Happy to follow up with a CI check (isort --check) and/or a pre-commit hook so it stays consistent without anyone thinking about it. if you'd prefer