Add exponential backoff retry for BEGIN IMMEDIATE transactions to handle
concurrent write load without spurious failures.
Changes:
- Add IsBusyError() helper to detect database locked errors
- Add beginImmediateWithRetry() with exponential backoff (10ms, 20ms, 40ms, 80ms, 160ms)
- Update CreateIssue and CreateIssuesInBatch to use retry logic
- Add comprehensive tests for error detection and retry behavior
- Handles context cancellation between retry attempts
- Fails fast on non-busy errors
This eliminates spurious SQLITE_BUSY failures under normal concurrent usage
while maintaining proper error handling for other failure modes.
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Co-Authored-By: Claude <noreply@anthropic.com>
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