perf(monitoring): exclude payloads from session summaries

Exclude message_content and variables from the message source used by
conversation session summaries. These fields were carried through window
sorting even though the resulting summaries did not use them.

Preserve complete payloads for message detail queries. Add regression
coverage for summary counts, session metadata, and full message retrieval.

src/langbot/pkg/api/http/service/monitoring_execution_details.py

@@ -100,7 +100,7 @@ def transcript_item(row):
 
 
 class ExecutionDetailsMixin:
-    def _session_message_source(self, workspace, bot_ids):
+    def _session_message_source(self, workspace, bot_ids, *, summary_only=False):
         """Combine delivered messages with ingress missing from the legacy ledger."""
         message = models.MonitoringMessage
         conversation = sa.func.coalesce(
@@ -148,7 +148,11 @@ class ExecutionDetailsMixin:
             'event_id': EventLog.event_id,
             'run_id': EventLog.run_id,
         }
-        columns = list(message.__table__.columns)
+        columns = [
+            column
+            for column in message.__table__.columns
+            if not summary_only or column.name not in {'message_content', 'variables'}
+        ]
         stored = sa.select(*columns).where(message.workspace_uuid == workspace, message.bot_id.in_(bot_ids))
         incoming = sa.select(
             *[values.get(column.name, sa.literal(None)).label(column.name) for column in columns]
@@ -184,7 +188,9 @@ class ExecutionDetailsMixin:
         self, workspace, bot_ids, start_time, end_time, user_query, is_active, limit, offset
     ):
         """Read session summaries across processor types without mutating history."""
-        messages = self._session_message_source(workspace, bot_ids)
+        # Window functions sort and retain their input rows. Carrying historical
+        # inline media through that sort can consume gigabytes for a summary.
+        messages = self._session_message_source(workspace, bot_ids, summary_only=True)
         key = [messages.c.workspace_uuid, messages.c.bot_id, messages.c.session_id]
         ranked = (
             sa.select(