A scan covering multiple targets inside a single API task (crawl,
'-m' bulk file, forms) ran every target through one StdDbOut keyed by
the single taskid, so _storeData()'s COMPLETE branch deleted each
earlier target's TARGET/TECHNIQUES rows - /scan/<id>/data returned
only the last target's results.
Mirror the --report-json fix for #6123: controller.start() now sets
kb.reportTaskId per target for API scans too, StdDbOut.write() keys
data rows '<taskid>-<n>' per target, and scan_data() folds the
per-target rows back into the task's flat result in target order.
Single-target scans keep the exact same response shape.
Regression tests in tests/test_api.py (TestStdDbOutMultiTarget,
TestScanDataMultiTarget).
