feat: Detect High-Frequency Contract Calls

[bellabuks]

What

Implements high-frequency contract call detection requested in #150, under src/modules/detection/contracts/.

Why

Exploits often generate abnormal transaction volume against a contract before or during an attack. This gives Sentinel a way to flag that volume in near real time.

Implementation

HighFrequencyContractCallsService runs two independent detection rules on every recorded call:

• Frequency monitoring — recordCall() tracks calls per contract in a sliding, tumbling window (getCallCount() / getWindowHistory() expose the current and recently-closed window counts).
• Baseline comparison — a rolling average is kept across the last baselineWindowCount closed windows (getBaselineAverage()). If the current window's count exceeds that average by baselineSpikeMultiplierx, a baseline_spike alert fires — this catches sustained-but-abnormal activity even when it stays under the absolute cap.
• Alert generation — onAlert() lets subscribers observe FrequencyAlerts (with an unsubscribe function), each carrying the triggering reason, call count, threshold, and (for baseline alerts) the baseline average.
• Configurable thresholds — a default FrequencyThresholdConfig (window size, absolute cap, baseline window count, spike multiplier) can be set via the constructor, and overridden per contract with setThreshold()/getThreshold(). clearContract() resets all tracked state for a contract.

Follows the same plain-class-plus-interfaces style already used by src/modules/detection/malicious-addresses.

Testing

15 new unit tests in high-frequency-calls.service.spec.ts, using explicit event timestamps (no real-time waiting or fake timers needed) covering: per-contract threshold configuration and overrides, window rolling/history capping, absolute-threshold alerts, baseline-spike alerts, no-alert-on-normal-activity, subscriber notification/unsubscription, and state reset via clearContract.

Verified locally:

• npm run lint — clean
• npm run format:check — clean for these files
• npm run build — passes
• npx jest --config jest.backend.config.js src/modules/detection/contracts — 15/15 passing
• Full src/modules suite — 70/70 passing (no regressions)

Acceptance Criteria

• Activity spikes detected
• Alerts generated
• Thresholds configurable

Closes #150

[mijinummi]

LGTM