fix: OPTIC-1407: Optimize the tasks API paginated prediction and annotation totals

[bmartel]

PR fulfills these requirements

• Commit message(s) and PR title follows the format [fix|feat|ci|chore|doc]: TICKET-ID: Short description of change made ex. fix: DEV-XXXX: Removed inconsistent code usage causing intermittent errors
• Tests for the changes have been added/updated (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)
• Best efforts were made to ensure docs/code are concise and coherent (checked for spelling/grammatical errors, commented out code, debug logs etc.)
• Self-reviewed and ran all changes on a local instance (for bug fixes/features)

Change has impacts in these area(s)

(check all that apply)

• Product design
• Backend (Database)
• Backend (API)
• Frontend

Describe the reason for change

At larger counts of tasks and annotations, these 2 queries would become incredibly inefficient and cause slowdowns. This PR instead uses the task column values to do aggregation and simple SUM's on the values to produce the same output.

Does this change affect performance?

Improves performance about 15-20x when looking at volumes of data where there are greater than 500K tasks, and 500K annotations per project.

What alternative approaches were there?

• Moving the original subquery to a JOIN, which is difficult to do without dropping down to raw SQL.
• Similarily this query while an improvement could possibly benefit further from moving this subquery to a JOIN. Same point of why not, it would likely require dropping out of the ORM to achieve as it really likes to use subqueries (which do tend to be faster in most cases, just not at certain scale)

What feature flags were used to cover this change?

• fflag_fix_back_optic_1407_optimize_tasks_api_pagination_counts

Does this PR introduce a breaking change?

(check only one)

• Yes, and covered entirely by feature flag(s)
• Yes, and covered partially by feature flag(s)
• No
• Not sure (briefly explain the situation below)

What level of testing was included in the change?

(check all that apply)

• e2e
• integration
• unit

Which logical domain(s) does this change affect?

Tasks List API

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[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 76.70%. Comparing base (e5c1339) to head (c730e49).
Report is 5 commits behind head on develop.

Files with missing lines	Patch %	Lines
label_studio/data_manager/api.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #6735      +/-   ##
===========================================
- Coverage    76.73%   76.70%   -0.04%     
===========================================
  Files          170      170              
  Lines        13885    13919      +34     
===========================================
+ Hits         10655    10676      +21     
- Misses        3230     3243      +13     

Flag	Coverage Δ
pytests	76.70% <90.00%> (-0.04%)	⬇️

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