feat: unread message tracking with blue dot indicator

Add ContactThreadRead model to track when users last viewed each contact thread.
Contacts with messages newer than the last read time show a blue dot in the sidebar.
Opening a thread automatically marks it as read via markThreadRead mutation.

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

frontend/prisma/schema.prisma

@@ -88,6 +88,7 @@ model Team {
   clientTimelineEntries ClientTimelineEntry[]
   contactInboxes      ContactInbox[]
   contactInboxPreferences ContactInboxPreference[]
+  contactThreadReads  ContactThreadRead[]
 }
 
 model User {
@@ -103,6 +104,7 @@ model User {
   aiConversations AiConversation[] @relation("ConversationCreator")
   aiMessages      AiMessage[]      @relation("ChatAuthor")
   contactInboxPreferences ContactInboxPreference[]
+  contactThreadReads      ContactThreadRead[]
 }
 
 model TeamMember {
@@ -142,10 +144,28 @@ model Contact {
   omniIdentities OmniContactIdentity[]
   contactInboxes ContactInbox[]
   clientTimelineEntries ClientTimelineEntry[]
+  contactThreadReads    ContactThreadRead[]
 
   @@index([teamId, updatedAt])
 }
 
+model ContactThreadRead {
+  id        String   @id @default(cuid())
+  teamId    String
+  userId    String
+  contactId String
+  readAt    DateTime @default(now())
+  createdAt DateTime @default(now())
+  updatedAt DateTime @updatedAt
+
+  team    Team    @relation(fields: [teamId], references: [id], onDelete: Cascade)
+  user    User    @relation(fields: [userId], references: [id], onDelete: Cascade)
+  contact Contact @relation(fields: [contactId], references: [id], onDelete: Cascade)
+
+  @@unique([userId, contactId])
+  @@index([teamId, userId])
+}
+
 model ContactNote {
   id        String   @id @default(cuid())
   contactId String   @unique