feat: 实现 sessions 持久化，支持会话恢复

backend/migrations/003_sessions.up.sql

@@ -0,0 +1,11 @@
+CREATE TABLE IF NOT EXISTS sessions (
+    id           UUID PRIMARY KEY,
+    user_id      UUID NOT NULL,
+    title        VARCHAR(256) NOT NULL DEFAULT '新对话',
+    config       JSONB NOT NULL DEFAULT '{}',
+    created_at   TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at   TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_sessions_user_id ON sessions (user_id);
+CREATE INDEX IF NOT EXISTS idx_sessions_user_updated ON sessions (user_id, updated_at DESC);