refactor: replace comment read receipts with per-task cursors

[b0ink]

Actual unit 12 measurements:

Metric	Old receipts	New cursors
Rows	25,751,861	2,285,027
Rows per task	823	73
Reduction	—	91.1%
Improvement	—	11.3× fewer rows

Table	Data + indexes
Old receipts	5.29 GB
New cursors	632 MB
Reduction	88.1%

TODO: Automated comments break this cursor design - OnTrack should have a "SYSTEM" user account that all automated comments send from. Eg. overseer assessment reports, task status updates (only automated task status updates?), project plan comments? anything that currently automatically gets marked as read by the receipient should be from the SYSTEM

Task comments now have a "audience" field, used to identify who the comment is intended for. "Students" are the audience for automated comments, "noone" for status updates, and either "staff" or "students" for text comments depending on the recipient. This allows the comment cursor to display up to a student's text comment, and the inbox will ignore any status updates, project plan changes, overseer assessments that would otherwise normally be marked as read by all parties

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
1 Accepted issue

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud