feat: LEAP-1602: prereq for updating existing locks

[jombooth]

Supersedes #6583 - created_at is needed for implementation of task expiry updating function, and this version is safer than using auto_now_add on a new field for an existing table. The sql/redis migration has been taken directly from that PR

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[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 76.73%. Comparing base (656fdfd) to head (ef99009).
Report is 10 commits behind head on develop.

Files with missing lines	Patch %	Lines
label_studio/tasks/models.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #6590   +/-   ##
========================================
  Coverage    76.73%   76.73%           
========================================
  Files          169      169           
  Lines        13819    13826    +7     
========================================
+ Hits         10604    10610    +6     
- Misses        3215     3216    +1     

Flag	Coverage Δ
pytests	76.73% <85.71%> (+<0.01%)	⬆️

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[triklozoid]

It doesn't fit requirement: