feat: 足迹 Footprints 个人留痕系统

桌面应用(Tauri+React+SQLite)：时间线/记录/学年学科成绩梯度/统计/类型与字段模板/附件内容寻址/详情视图/PWA

同步服务端(sync-server)：多用户+邀请码/LWW推拉/seq游标/附件去重/Docker部署

多端同步地基：uuid v7/HLC混合逻辑时钟/软删除墓碑/sync_oplog，Node与Rust双端一致并通过自动化测试

sync-server/src/db.mjs

@@ -0,0 +1,91 @@
+// 同步服务存储层：SQLite（node:sqlite 内置，零依赖）
+import { DatabaseSync } from 'node:sqlite';
+import fs from 'node:fs';
+import path from 'node:path';
+
+export const DATA_DIR = process.env.DATA_DIR || path.join(process.cwd(), 'data');
+export const BLOB_DIR = path.join(DATA_DIR, 'blobs');
+const DB_PATH = path.join(DATA_DIR, 'sync.db');
+
+let db = null;
+
+export function getDb() {
+  if (db) return db;
+  fs.mkdirSync(BLOB_DIR, { recursive: true });
+  db = new DatabaseSync(DB_PATH);
+  db.exec('PRAGMA journal_mode = WAL;');
+  db.exec('PRAGMA foreign_keys = ON;');
+  db.exec(`
+CREATE TABLE IF NOT EXISTS users (
+  id TEXT PRIMARY KEY,
+  username TEXT NOT NULL UNIQUE,
+  password_hash TEXT NOT NULL,
+  salt TEXT NOT NULL,
+  created_at TEXT NOT NULL DEFAULT (datetime('now','localtime'))
+);
+
+CREATE TABLE IF NOT EXISTS devices (
+  id TEXT PRIMARY KEY,
+  user_id TEXT NOT NULL REFERENCES users(id),
+  name TEXT NOT NULL,
+  last_seen_at TEXT NOT NULL DEFAULT (datetime('now','localtime')),
+  UNIQUE(user_id, name)
+);
+
+CREATE TABLE IF NOT EXISTS tokens (
+  token TEXT PRIMARY KEY,
+  user_id TEXT NOT NULL REFERENCES users(id),
+  device_id TEXT NOT NULL REFERENCES devices(id),
+  created_at TEXT NOT NULL DEFAULT (datetime('now','localtime')),
+  expires_at TEXT NOT NULL
+);
+
+-- 每行最新状态（按 user + table + row_uuid 唯一，LWW 胜者）
+CREATE TABLE IF NOT EXISTS current (
+  user_id TEXT NOT NULL,
+  table_name TEXT NOT NULL,
+  row_uuid TEXT NOT NULL,
+  op TEXT NOT NULL,
+  hlc TEXT NOT NULL,
+  ms INTEGER NOT NULL,
+  cnt INTEGER NOT NULL,
+  dev TEXT NOT NULL,
+  data TEXT,
+  updated_seq INTEGER,
+  PRIMARY KEY (user_id, table_name, row_uuid)
+);
+
+-- 全量变更历史（seq 单调递增，作为拉取游标基准）
+CREATE TABLE IF NOT EXISTS changes (
+  seq INTEGER PRIMARY KEY AUTOINCREMENT,
+  user_id TEXT NOT NULL,
+  table_name TEXT NOT NULL,
+  row_uuid TEXT NOT NULL,
+  op TEXT NOT NULL,
+  hlc TEXT NOT NULL,
+  data TEXT,
+  created_at TEXT NOT NULL DEFAULT (datetime('now','localtime'))
+);
+
+CREATE INDEX IF NOT EXISTS idx_current_seq ON current(user_id, updated_seq);
+CREATE INDEX IF NOT EXISTS idx_changes_user ON changes(user_id, seq);
+
+-- 附件：内容寻址去重
+CREATE TABLE IF NOT EXISTS attachments (
+  user_id TEXT NOT NULL,
+  sha256 TEXT NOT NULL,
+  size INTEGER NOT NULL,
+  mime TEXT,
+  file_name TEXT,
+  created_at TEXT NOT NULL DEFAULT (datetime('now','localtime')),
+  PRIMARY KEY (user_id, sha256)
+);
+`);
+  return db;
+}
+
+// 允许同步的表（与客户端 schema 对应；footprint_dimension 以 uuid 对作普通行同步）
+export const SYNC_TABLES = new Set([
+  'footprint', 'footprint_type', 'taxonomy', 'dimension', 'attachment',
+  'semester', 'course_grade', 'metric_def', 'saved_view', 'footprint_dimension'
+]);