Backend/i18n: Support localized datetime formats

[tristanlabelle]

We only supported one localized datetime format. This isn't quite enough in practice. For example, when we send a push notification about a host request, saying "May 16, 2026" is not ideal: the year is implicit, and it'd be useful to have the day of the week.

This implements multiple datetime formats with an API similar to the frontend's. Python doesn't have Javascript's convenient Intl.DateTimeFormat, but the Babel library exposes everything from the Unicode CLDR that we need to implement the functionality, even if it gets a little a little gnarly. Hopefully I documented it well.

I also changed the defaults a little bit to use long-form month names and omit seconds (both unless requested). This aligns with the frontend and is hopefully less surprising.

I also gave LocalizationContext the job of resolving the babel.Locale, as to not have to do it on every formatting operation.

I also leveraged the new feature to tweak datetime formats in push notifications for events and host requests. I didn't change those in emails since I'm working on email changes in parallel.

Fixes #7885, #8490

Testing

Updated and added tests.

Backend checklist

• Added tests for any new code or added a regression test if fixing a bug
• Run the backend locally and it works
• Added migrations if there are any database changes, rebased onto develop if necessary for linear migration history

For maintainers

• Maintainers can push commits to my branch
• Maintainers can merge this PR for me

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Project	Deployment	Actions	Updated (UTC)
couchers	Ready	Preview	May 26, 2026 8:16pm

[aapeliv]

i'll trust you on this one

[tristanlabelle]

Rebased to merge with the feature flags change.

[github-actions]

📝 Release Notes

This PR should be included in the release notes.

Suggested release note:

Improved localized date and time formatting in push notifications to make event and host request details clearer and more natural

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: include
Reasoning: This PR mainly improves how dates and times are formatted in notifications, which is a noticeable user-facing polish improvement: push notifications for events and host requests now use more natural localized formats, including omitting unnecessary years and adding weekdays where helpful. While much of the work is backend i18n infrastructure, it results in clearer notifications that users will see.