feat: Implement air-gap functionality with timeline impact and evidence snapshot services

- Added AirgapTimelineImpact, AirgapTimelineImpactInput, and AirgapTimelineImpactResult records for managing air-gap bundle import impacts.
- Introduced EvidenceSnapshotRecord, EvidenceSnapshotLinkInput, and EvidenceSnapshotLinkResult records for linking findings to evidence snapshots.
- Created IEvidenceSnapshotRepository interface for managing evidence snapshot records.
- Developed StalenessValidationService to validate staleness and enforce freshness thresholds.
- Implemented AirgapTimelineService for emitting timeline events related to bundle imports.
- Added EvidenceSnapshotService for linking findings to evidence snapshots and verifying their validity.
- Introduced AirGapOptions for configuring air-gap staleness enforcement and thresholds.
- Added minimal jsPDF stub for offline/testing builds in the web application.
- Created TypeScript definitions for jsPDF to enhance type safety in the web application.

src/Findings/StellaOps.Findings.Ledger/Observability/LedgerMetrics.cs

@@ -59,6 +59,21 @@ internal static class LedgerMetrics
         "ledger_attachments_encryption_failures_total",
         description: "Count of attachment encryption/signing/upload failures.");
 
+    private static readonly Histogram<double> AirgapStalenessSeconds = Meter.CreateHistogram<double>(
+        "ledger_airgap_staleness_seconds",
+        unit: "s",
+        description: "Current staleness of air-gap imported data by domain.");
+
+    private static readonly Counter<long> StalenessValidationFailures = Meter.CreateCounter<long>(
+        "ledger_staleness_validation_failures_total",
+        description: "Count of staleness validation failures blocking exports.");
+
+    private static readonly ObservableGauge<double> AirgapStalenessGauge =
+        Meter.CreateObservableGauge("ledger_airgap_staleness_gauge_seconds", ObserveAirgapStaleness, unit: "s",
+            description: "Current staleness of air-gap data by domain.");
+
+    private static readonly ConcurrentDictionary<string, double> AirgapStalenessByDomain = new(StringComparer.Ordinal);
+
     private static readonly ObservableGauge<double> ProjectionLagGauge =
         Meter.CreateObservableGauge("ledger_projection_lag_seconds", ObserveProjectionLag, unit: "s",
             description: "Lag between ledger recorded_at and projection application time.");
@@ -228,6 +243,27 @@ internal static class LedgerMetrics
     public static void RecordProjectionLag(TimeSpan lag, string? tenantId) =>
         UpdateProjectionLag(tenantId, lag.TotalSeconds);
 
+    public static void RecordAirgapStaleness(string? domainId, long stalenessSeconds)
+    {
+        var key = string.IsNullOrWhiteSpace(domainId) ? "unknown" : domainId;
+        var tags = new KeyValuePair<string, object?>[]
+        {
+            new("domain", key)
+        };
+        AirgapStalenessSeconds.Record(stalenessSeconds, tags);
+        AirgapStalenessByDomain[key] = stalenessSeconds;
+    }
+
+    public static void RecordStalenessValidationFailure(string? domainId)
+    {
+        var key = string.IsNullOrWhiteSpace(domainId) ? "unknown" : domainId;
+        var tags = new KeyValuePair<string, object?>[]
+        {
+            new("domain", key)
+        };
+        StalenessValidationFailures.Add(1, tags);
+    }
+
     private static IEnumerable<Measurement<double>> ObserveProjectionLag()
     {
         foreach (var kvp in ProjectionLagByTenant)
@@ -267,6 +303,14 @@ internal static class LedgerMetrics
             new KeyValuePair<string, object?>("git_sha", GitSha));
     }
 
+    private static IEnumerable<Measurement<double>> ObserveAirgapStaleness()
+    {
+        foreach (var kvp in AirgapStalenessByDomain)
+        {
+            yield return new Measurement<double>(kvp.Value, new KeyValuePair<string, object?>("domain", kvp.Key));
+        }
+    }
+
     private static string NormalizeRole(string role) => string.IsNullOrWhiteSpace(role) ? "unspecified" : role.ToLowerInvariant();
 
     private static string NormalizeTenant(string? tenantId) => string.IsNullOrWhiteSpace(tenantId) ? string.Empty : tenantId;