feat(archive): month snapshots on the same node, drop extra-node and HTML /view.

Fixes EventHub/EventHubBack#76

Co-authored-by: Cursor <cursoragent@cursor.com>

src/migrations/20260817140000_month_snapshot.erl

@@ -0,0 +1,41 @@
+%% @doc month_snapshot table (Back#76). disc_only — blobs not in RAM.
+-module('20260817140000_month_snapshot').
+
+-export([up/0, down/0]).
+
+-include("records.hrl").
+
+up() ->
+  ensure_table(),
+  ensure_index(month_snapshot, calendar_id),
+  ensure_index(month_snapshot, year_month),
+  ok.
+
+down() ->
+  _ = mnesia:delete_table(month_snapshot),
+  ok.
+
+ensure_table() ->
+  case lists:member(month_snapshot, mnesia:system_info(tables)) of
+    true ->
+      ok;
+    false ->
+      Attrs = record_info(fields, month_snapshot),
+      case mnesia:create_table(month_snapshot, [
+             {disc_only_copies, [node()]},
+             {attributes, Attrs}
+           ]) of
+        {atomic, ok} -> ok;
+        {aborted, {already_exists, month_snapshot}} -> ok;
+        {aborted, Reason} -> error({create_table_failed, month_snapshot, Reason})
+      end
+  end.
+
+ensure_index(Table, Attr) ->
+  case mnesia:add_table_index(Table, Attr) of
+    {atomic, ok} -> ok;
+    {aborted, {already_exists, Table, _Pos}} -> ok;
+    {aborted, {already_exists, Table, Attr}} -> ok;
+    {aborted, {already_exists, _}} -> ok;
+    {aborted, Reason} -> error({add_index_failed, Table, Attr, Reason})
+  end.