Add aggregator backfill, pulling fine+coarse buckets from collectors

proto/logtail.proto

@@ -99,8 +99,15 @@ message Snapshot {
   string             source    = 1;
   int64              timestamp = 2;
   repeated TopNEntry entries   = 3; // top-50K for this 1-minute bucket, sorted desc
+  bool               is_coarse = 4; // true for coarse-ring (5-min) buckets in DumpSnapshots
 }
 
+// DumpSnapshots — returns all ring buffer contents for backfill on aggregator restart.
+// Streams fine-ring buckets (is_coarse=false) followed by coarse-ring buckets
+// (is_coarse=true), then closes. The lock is held only for the initial copy.
+
+message DumpSnapshotsRequest {}
+
 // ListTargets — returns the targets this node knows about.
 // The aggregator returns all configured collectors; a collector returns itself.
 
@@ -120,4 +127,5 @@ service LogtailService {
   rpc Trend           (TrendRequest)        returns (TrendResponse);
   rpc StreamSnapshots (SnapshotRequest)     returns (stream Snapshot);
   rpc ListTargets     (ListTargetsRequest)  returns (ListTargetsResponse);
+  rpc DumpSnapshots   (DumpSnapshotsRequest) returns (stream Snapshot);
 }