fix: optimize project queue total count

[fredheir]

Reason for change

ProjectSerializer.get_queue_total currently combines a negated annotation relation, a positive annotation relation, and distinct().count(). Django compiles that to a duplicate-producing outer join plus an anti-subquery and DISTINCT. This PR expresses the same membership rule directly as unlabeled OR EXISTS(current-user annotation).

Fixes #9861.

Performance evidence

Read-only PostgreSQL A/B tests on three representative project/user pairs preserved exact counts and reduced server execution time:

• 268.2 ms -> 16.3 ms
• 31.5 ms -> 9.5 ms
• 35.9 ms -> 7.5 ms

The PostgreSQL ORM SQL for the candidate uses one correlated EXISTS, with no outer annotation join or DISTINCT.

Testing

• projects/tests/test_serializers.py: 17 passed on SQLite
• Ruff check and format: passed
• Regression matrix covers unlabeled tasks, current-user annotations, other-user annotations, and mixed annotations
• Test asserts one count query, EXISTS present, and DISTINCT absent
• Live PostgreSQL exact-result parity verified for all three A/B cases

Acceptance criteria

• Exact task-membership parity with the existing predicate
• One count query
• Correlated EXISTS; no outer annotation join or DISTINCT
• SQLite and PostgreSQL-compatible ORM query

Rollout and risk

No schema or API contract change is involved. The risk is limited to task-membership equivalence; the regression matrix and live exact-result comparisons cover that boundary. No feature flag is needed.

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