feat: add session_messages table for conversation history

Add a session_messages table that stores the full user/assistant
conversation history within a session. Each session request now
persists both the user message and assistant response atomically
alongside the session_request and balance update.

- New schema: session_messages (id, session_id, role, content,
  session_request_id, created_at) with index on session_id
- New migration: 0008_session_messages.sql
- New endpoint: GET /sessions/:id/messages (macaroon-authed)
- Messages inserted transactionally during POST /sessions/:id/request

Fixes #37

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/db/migrations/0008_session_messages.sql

@@ -0,0 +1,13 @@
+-- Migration: Session messages for conversation history
+-- Stores user/assistant message pairs produced during session requests.
+
+CREATE TABLE IF NOT EXISTS session_messages (
+  id                 SERIAL PRIMARY KEY,
+  session_id         TEXT NOT NULL REFERENCES sessions(id),
+  role               TEXT NOT NULL,
+  content            TEXT NOT NULL,
+  session_request_id TEXT,
+  created_at         TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX idx_session_messages_session_id ON session_messages(session_id);

lib/db/src/schema/index.ts

@@ -5,6 +5,7 @@ export * from "./messages";
 export * from "./bootstrap-jobs";
 export * from "./world-events";
 export * from "./sessions";
+export * from "./session-messages";
 export * from "./nostr-identities";
 export * from "./timmy-config";
 export * from "./free-tier-grants";

lib/db/src/schema/session-messages.ts

@@ -0,0 +1,35 @@
+import { pgTable, text, timestamp, serial } from "drizzle-orm/pg-core";
+import { createInsertSchema } from "drizzle-zod";
+import { z } from "zod/v4";
+import { sessions } from "./sessions";
+
+// ── session_messages ─────────────────────────────────────────────────────────
+// Stores the full conversation history within a session.
+// Each session request produces a "user" message (the request text)
+// and an "assistant" message (the AI response), linked to the session
+// and optionally to the session_request that produced them.
+
+export const SESSION_MESSAGE_ROLES = ["user", "assistant"] as const;
+export type SessionMessageRole = (typeof SESSION_MESSAGE_ROLES)[number];
+
+export const sessionMessages = pgTable("session_messages", {
+  id: serial("id").primaryKey(),
+  sessionId: text("session_id")
+    .notNull()
+    .references(() => sessions.id),
+  role: text("role").$type<SessionMessageRole>().notNull(),
+  content: text("content").notNull(),
+
+  // Links back to the session_request that produced this message pair (nullable for flexibility)
+  sessionRequestId: text("session_request_id"),
+
+  createdAt: timestamp("created_at", { withTimezone: true }).defaultNow().notNull(),
+});
+
+export const insertSessionMessageSchema = createInsertSchema(sessionMessages).omit({
+  id: true,
+  createdAt: true,
+});
+
+export type SessionMessage = typeof sessionMessages.$inferSelect;
+export type InsertSessionMessage = z.infer<typeof insertSessionMessageSchema>;