⚡ Performance Optimization: Resolve N+1 Query in export_to_sql_by_form

[fderuiter]

💡 What:
Optimized the export_to_sql_by_form function in src/imednet/integrations/export.py to eliminate an N+1 query pattern. Instead of making a remote API call for each form to fetch its variables, the function now fetches all variables for the study in a single bulk request and groups them by form_id locally using a dictionary.

🎯 Why:
Previously, the loop executed sdk.variables.list(..., formId=form.form_id) for every form in the forms list. For studies with many forms, this resulted in significant I/O overhead and slow export performance. Bulk-fetching variables reduces the number of API calls from N+1 to 1 (plus the initial forms list call).

📊 Measured Improvement:

• Baseline: 5 API calls to sdk.variables.list for 5 forms.
• Optimized: 1 API call to sdk.variables.list for any number of forms.
• Change: Significant reduction in network latency and API overhead, especially for large studies.

Verified with a reproduction script that confirmed the call count reduction and ensured functionality remains intact. Static analysis with mypy passed successfully.

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PR created automatically by Jules for task 4238338433989229310 started by @fderuiter

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