feat: add task manager db calls

[stevenle]

Summary

• add Firestore utilities for creating tasks and tracking assignee updates
• support default task assignee storage in site settings and utilities for setting it
• add task comment APIs that preserve edit and delete history

Testing

• not run (not requested)

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Codex Task

[jeremydw]

LGTM - we might also want to add a status and CC fields.

BTW, I considered suggesting naming these "Issues" instead of "Tasks" but I think the term "Tasks" aligns better with our vision for how this will eventually be used. (GitHub Issues and Google Buganizer call them "Issues" hence the thought.)

[stevenle]

added status field, i'll probably refine this more once i start working on the ui (which i'll probably do by hand). cc field will need to come a little later when i figure out the email api, right now the tasks don't have a notification system so i'll need to figure that part out separately.