Add SourcemapDetectorHandler with atomic state transitions

[wedamija]

Implements the detector handler for sourcemap configuration issues. Extends StatefulDetectorHandler but overrides evaluate_impl to skip dedupe/threshold logic and use atomic filter().update() on DetectorState as a lock to prevent duplicate occurrence creation across concurrent processes. Resolution is handled separately by a periodic task.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Missing transaction.atomic() around create() catching IntegrityError

Medium Severity

The IntegrityError from DetectorState.objects.create() is caught without wrapping the call in transaction.atomic(). In PostgreSQL, a failed statement inside a transaction aborts the entire transaction. While this works in autocommit mode (current production path), if _try_state_transition is ever called inside a transaction.atomic() block — including Django's TestCase which wraps every test in one — the caught IntegrityError leaves the connection in a broken state, causing subsequent DB operations to fail with TransactionManagementError. The codebase already follows the correct pattern in processors/action.py, where IntegrityError is caught from a transaction.atomic() block.