feat: direct messages

1:1 conversations (userAId < userBId dedup pair), messages,
per-participant read tracking (conversation_reads). Block relationship
is enforced on every send. UI: /messages list + /messages/[id] thread
(5s poll), MessageButton on profiles, unread-badged nav icon.

This completes the social-feature backlog: notifications, rate
limiting, blocking, reporting, search/discovery, mentions, DMs, plus
fixes for the recipe-visibility 404, follow race, and 2-level comment
thread cap found during the earlier audit.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

packages/db/src/schema/index.ts

@@ -4,3 +4,4 @@ export * from "./social";
 export * from "./meal-planning";
 export * from "./tiers";
 export * from "./webhooks";
+export * from "./messaging";

packages/db/src/schema/messaging.ts

@@ -0,0 +1,53 @@
+import {
+  pgTable,
+  text,
+  timestamp,
+  index,
+  unique,
+  primaryKey,
+} from "drizzle-orm/pg-core";
+import { relations } from "drizzle-orm";
+import { users } from "./users";
+
+// 1:1 conversations only. userAId is always the lexicographically smaller
+// of the two participant ids, so (userAId, userBId) uniquely identifies a
+// pair regardless of who messaged first.
+export const conversations = pgTable("conversations", {
+  id: text("id").primaryKey(),
+  userAId: text("user_a_id").notNull().references(() => users.id, { onDelete: "cascade" }),
+  userBId: text("user_b_id").notNull().references(() => users.id, { onDelete: "cascade" }),
+  createdAt: timestamp("created_at").notNull().defaultNow(),
+  lastMessageAt: timestamp("last_message_at").notNull().defaultNow(),
+}, (t) => [
+  unique("conversations_pair_uniq").on(t.userAId, t.userBId),
+  index("conversations_last_message_idx").on(t.lastMessageAt),
+]);
+
+export const messages = pgTable("messages", {
+  id: text("id").primaryKey(),
+  conversationId: text("conversation_id").notNull().references(() => conversations.id, { onDelete: "cascade" }),
+  senderId: text("sender_id").notNull().references(() => users.id, { onDelete: "cascade" }),
+  content: text("content").notNull(),
+  createdAt: timestamp("created_at").notNull().defaultNow(),
+}, (t) => [
+  index("messages_conversation_idx").on(t.conversationId, t.createdAt),
+]);
+
+export const conversationReads = pgTable("conversation_reads", {
+  conversationId: text("conversation_id").notNull().references(() => conversations.id, { onDelete: "cascade" }),
+  userId: text("user_id").notNull().references(() => users.id, { onDelete: "cascade" }),
+  lastReadAt: timestamp("last_read_at").notNull().defaultNow(),
+}, (t) => [
+  primaryKey({ columns: [t.conversationId, t.userId] }),
+]);
+
+export const conversationsRelations = relations(conversations, ({ one, many }) => ({
+  userA: one(users, { fields: [conversations.userAId], references: [users.id], relationName: "userA" }),
+  userB: one(users, { fields: [conversations.userBId], references: [users.id], relationName: "userB" }),
+  messages: many(messages),
+}));
+
+export const messagesRelations = relations(messages, ({ one }) => ({
+  conversation: one(conversations, { fields: [messages.conversationId], references: [conversations.id] }),
+  sender: one(users, { fields: [messages.senderId], references: [users.id] }),
+}));