anomalyDetector: Add configurable sensitivityLevel option (low/medium/high)

[Kingsman-99]

Description

anomalyDetector flags unusual payment patterns but uses a hard-coded Z-score threshold. Contributors should expose a sensitivityLevel option ("low" | "medium" | "high") that maps to Z-score thresholds (3.0, 2.0, 1.5 respectively), allowing operators to tune false-positive rates for their user base.

Acceptance Criteria

• A sensitivityLevel: "low" | "medium" | "high" option is accepted (default "medium")
• Each level maps to the correct Z-score threshold (low→3.0, medium→2.0, high→1.5)
• The current sensitivityLevel is included in every anomaly report object for auditability
• Existing tests continue to pass

Context

• Target file: src/lib/anomalyDetector.ts