Backend/notifs: Make ChatMessage protos loc'able and port email

[tristanlabelle]

Migrating the chat message notification emails to the new email system and making them localizable.

• Update notifications_pb2.ChatMessage and ChatMissedMessage to split out the non-localizable message field into its constituents: group_chat_title and unseen_count. For backcompat with database entries containing old notifications, I parse these out of the message field if I have to.
• Migrated chat message notifications to new email system.

The ChatMissedMessages design is a little hacky because it includes a list of messages, but those are not all the missed messages, rather the last missed messages from all chats that have missed messages.

Note: I duplicated the view_action string for now but will do a pass to clean things up once I'm done porting emails.

Testing

Dumped emails with uv run dump-emails.

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 8:12pm

[tristanlabelle]

Rebased to merge the list of unhandled enums in the default case of the match statement in render_email.

[github-actions]

📝 Release Notes

This PR does not need to be included in release notes.

Reason: This PR mainly migrates chat notification emails to a new backend email system and makes them localizable. While users may eventually benefit from translated or more consistent chat emails, the change is mostly internal infrastructure work around email rendering and notification data, without a clearly visible new feature or major end-user behavior change described here.

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: exclude
Reasoning: This PR mainly migrates chat notification emails to a new backend email system and makes them localizable. While users may eventually benefit from translated or more consistent chat emails, the change is mostly internal infrastructure work around email rendering and notification data, without a clearly visible new feature or major end-user behavior change described here.