perf(iios): listThreads fetches last-message-per-thread in one query

Replaced the per-thread findFirst inside Promise.all (N parallel queries) with a
single Postgres DISTINCT ON query. A caller with many threads no longer fans out
and exhausts the connection pool (the conversation.list 500 under accumulated data).
message.spec 16/16 green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/iios-service/src/messaging/message.service.ts

@@ -208,13 +208,27 @@ export class MessageService {
       membersBy.set(p.threadId, [...(membersBy.get(p.threadId) ?? []), name]);
     }
 
-    const summaries = await Promise.all(
-      threads.map(async (t) => {
-        const last = await this.prisma.iiosInteraction.findFirst({
-          where: { threadId: t.id },
-          orderBy: { occurredAt: 'desc' },
-          include: { parts: { where: { kind: 'TEXT' }, take: 1 } },
-        });
+    // Latest message per thread in ONE query (Postgres DISTINCT ON) instead of one findFirst
+    // per thread — a caller with many threads no longer fans out N parallel queries and
+    // exhausts the connection pool.
+    const lastRows =
+      threads.length === 0
+        ? []
+        : await this.prisma.$queryRaw<Array<{ threadId: string; occurredAt: Date; lastMessage: string | null }>>(Prisma.sql`
+            SELECT DISTINCT ON (i."threadId")
+              i."threadId"   AS "threadId",
+              i."occurredAt" AS "occurredAt",
+              (SELECT p."bodyText" FROM "IiosMessagePart" p
+                 WHERE p."interactionId" = i.id AND p.kind::text = 'TEXT'
+                 ORDER BY p."partIndex" ASC LIMIT 1) AS "lastMessage"
+            FROM "IiosInteraction" i
+            WHERE i."threadId" IN (${Prisma.join(threads.map((t) => t.id))})
+            ORDER BY i."threadId", i."occurredAt" DESC
+          `);
+    const lastBy = new Map(lastRows.map((r) => [r.threadId, r]));
+
+    const summaries = threads.map((t) => {
+      const last = lastBy.get(t.id);
       const members = membersBy.get(t.id) ?? [];
       return {
         threadId: t.id,
@@ -225,11 +239,10 @@ export class MessageService {
         participantCount: members.length,
         unread: unreadBy.get(t.id) ?? 0,
         muted: mutedBy.get(t.id) ?? false,
-          lastMessage: last?.parts[0]?.bodyText ?? undefined,
+        lastMessage: last?.lastMessage ?? undefined,
         lastAt: last?.occurredAt,
       };
-      }),
-    );
+    });
     return summaries.sort((a, b) => (b.lastAt?.getTime() ?? 0) - (a.lastAt?.getTime() ?? 0));
   }