feat: member portal Sprint 2 — digest history page and restore Digest/DigestConfig schema models

- Restore Digest + DigestConfig models to schema.prisma (wiped by prior git reset)
- Add Tenant.digestConfig / Tenant.digests relations
- GET /my/digest API endpoint (last 20 digests, truncated summary)
- BFF route /api/my/digest
- /my/digest page: digest cards with date, message count, summary preview
- Add Digest nav item to MemberShell sidebar

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/api/prisma/schema.prisma

@@ -88,6 +88,8 @@ model Tenant {
   auditEvents    AuditEvent[]
   rules          TenantRule[]
   groupAccesses  GroupAccess[]
+  digestConfig   DigestConfig?
+  digests        Digest[]
 }
 
 enum AdminRole {
@@ -495,3 +497,34 @@ model TenantRule {
   @@index([tenantId, isActive])
   @@index([tenantId, matchType])
 }
+
+// ============================================================================
+// Digest
+// ============================================================================
+
+model DigestConfig {
+  id              String   @id @default(cuid())
+  tenantId        String   @unique
+  tenant          Tenant   @relation(fields: [tenantId], references: [id])
+  targetGroupJid  String
+  targetAccountId String
+  scheduleHour    Int      @default(20)
+  scheduleMinute  Int      @default(0)
+  isActive        Boolean  @default(true)
+  lastSentAt      DateTime?
+  createdAt       DateTime @default(now())
+  updatedAt       DateTime @updatedAt
+}
+
+model Digest {
+  id         String   @id @default(cuid())
+  tenantId   String
+  tenant     Tenant   @relation(fields: [tenantId], references: [id])
+  content    String
+  messageIds String[]
+  digestDate DateTime
+  sentAt     DateTime @default(now())
+
+  @@unique([tenantId, digestDate])
+  @@index([tenantId])
+}