⚡ Bolt: Remove redundant query in _build_issues_batch

[tachyon-beep]

💡 What: Removed an extra batched database query (COUNT(*)) for open blockers during the creation of the Issue model. Replaced open_blockers_by_id with a check against len(blocked_by_id).
🎯 Why: Step 4 of the _build_issues_batch function already gathers all open blockers for an issue using the identical blocker condition constraints. Step 6 then performed an extra query to merely get the count of these elements. Eliminating this query reduces database load and unnecessary SQLite queries inside Python loops without losing any logic.
📊 Impact: Reduces batched db queries by 1 per batch execution, optimizing issue fetching logic and avoiding an N+1 scaling bottleneck for larger operations.
🔬 Measurement: Verify changes by checking backend test cases or running the full test suite (uv run pytest tests/). Execution logic remains completely untouched and tests pass securely.

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PR created automatically by Jules for task 9287087697125375119 started by @tachyon-beep

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