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);