fix(backend): declare LastIngestAt in the SessionsIndex manifest

bbc7a1d8 stamped LastIngestAt into IncrementSessionEventCountAsync's merge
entity intending it as Sessions-only ("deliberately not mirrored"), but that
same entity flows through SyncSessionIndexAsync into the index merge — the
field was already landing in SessionsIndex on every ingest batch, just
undeclared. The fail-soft drift guard has been warning ~1,200x/h in prod
since the 08-20 deploy, and a StartedAt-shift full rebuild would silently
drop the column, making index-served session reads lose lastIngestAt that
Sessions-served reads carry.

Declare it properly instead of suppressing: ConditionallyProjected in
SessionIndexFieldManifest, projected in BuildSessionIndexEntity (read side
MapToSessionSummary already handled it), sample value pinned in the
bidirectional manifest tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/Backend/AutopilotMonitor.Functions.Tests/SessionIndexFieldManifestTests.cs

@@ -90,6 +90,7 @@ public class SessionIndexFieldManifestTests
         ["DiagnosticsBlobName"] = "diag-0001.zip",
         ["DiagnosticsBlobDestination"] = "Hosted",
         ["LastEventAt"] = new DateTimeOffset(StartedAt.AddMinutes(44)),
+        ["LastIngestAt"] = new DateTimeOffset(StartedAt.AddMinutes(46)),
         ["ResumedAt"] = new DateTimeOffset(StartedAt.AddMinutes(10)),
         ["StalledAt"] = new DateTimeOffset(StartedAt.AddMinutes(20)),
         ["AvgApiLatencyMs"] = 513.5,

src/Backend/AutopilotMonitor.Functions/Services/SessionIndexFieldManifest.cs

@@ -78,6 +78,7 @@ namespace AutopilotMonitor.Functions.Services
             "DiagnosticsBlobName",
             "DiagnosticsBlobDestination",
             "LastEventAt",
+            "LastIngestAt",
             "ResumedAt",
             "StalledAt",
             "AvgApiLatencyMs",

src/Backend/AutopilotMonitor.Functions/Services/TableStorageService.Sessions.cs

@@ -225,6 +225,13 @@ namespace AutopilotMonitor.Functions.Services
             if (lastEventAt.HasValue)
                 indexEntity["LastEventAt"] = EnsureUtc(lastEventAt.Value);
 
+            // Server-frame ingest stamp (IncrementSessionEventCountAsync merges this on every
+            // batch, so the index already carries it) — must be in this superset or a
+            // StartedAt-shift full upsert drops it and index-served reads lose the field.
+            var lastIngestAt = sessionEntity.GetDateTimeOffset("LastIngestAt")?.UtcDateTime;
+            if (lastIngestAt.HasValue)
+                indexEntity["LastIngestAt"] = EnsureUtc(lastIngestAt.Value);
+
             var resumedAt = sessionEntity.GetDateTimeOffset("ResumedAt")?.UtcDateTime;
             if (resumedAt.HasValue)
                 indexEntity["ResumedAt"] = EnsureUtc(resumedAt.Value);