feat: 客户端加密 encrypted 字段，数据库只存密文 (v0.5.0)

- 新增 src/crypto.rs：AES-256-GCM 加解密 + Argon2id 密钥派生 + OS Keychain 读写
- 新增 `secrets init` 命令：输入 Master Password，派生 Master Key 存入 Keychain
- 新增 `secrets migrate-encrypt` 命令：将旧明文 JSONB 数据批量加密
- 修改 db.rs：encrypted 列 JSONB → BYTEA，新增 kv_config 表（存 Argon2id salt）
- 修改 models.rs：encrypted 字段类型 Value → Vec<u8>
- 修改 add/update：写入前 encrypt_json，update 读取后 decrypt → 合并 → 重新加密
- 修改 search：按需解密，未解密时显示 _encrypted:true/_key_count:N
- 通过 6 个 crypto 单元测试（加解密、JSON roundtrip、Argon2id 确定性）

Made-with: Cursor

src/db.rs

@@ -23,7 +23,7 @@ pub async fn migrate(pool: &PgPool) -> Result<()> {
             name        VARCHAR(256) NOT NULL,
             tags        TEXT[]       NOT NULL DEFAULT '{}',
             metadata    JSONB        NOT NULL DEFAULT '{}',
-            encrypted   JSONB        NOT NULL DEFAULT '{}',
+            encrypted   BYTEA        NOT NULL DEFAULT '\x',
             created_at  TIMESTAMPTZ  NOT NULL DEFAULT NOW(),
             updated_at  TIMESTAMPTZ  NOT NULL DEFAULT NOW(),
             UNIQUE(namespace, kind, name)
@@ -35,11 +35,37 @@ pub async fn migrate(pool: &PgPool) -> Result<()> {
         EXCEPTION WHEN OTHERS THEN NULL;
         END $$;
 
+        -- Migrate encrypted column from JSONB to BYTEA if still JSONB type.
+        -- After migration, old plaintext rows will have their JSONB data
+        -- stored as raw bytes (not yet re-encrypted); run `secrets migrate-encrypt`
+        -- to encrypt them with the master key.
+        DO $$ BEGIN
+            IF EXISTS (
+                SELECT 1 FROM information_schema.columns
+                WHERE table_name = 'secrets'
+                  AND column_name = 'encrypted'
+                  AND data_type = 'jsonb'
+            ) THEN
+                ALTER TABLE secrets RENAME COLUMN encrypted TO encrypted_jsonb_old;
+                ALTER TABLE secrets ADD COLUMN encrypted BYTEA NOT NULL DEFAULT '\x';
+                -- Copy existing JSONB data as raw UTF-8 bytes so nothing is lost
+                UPDATE secrets SET encrypted = convert_to(encrypted_jsonb_old::text, 'UTF8');
+                ALTER TABLE secrets DROP COLUMN encrypted_jsonb_old;
+            END IF;
+        EXCEPTION WHEN OTHERS THEN NULL;
+        END $$;
+
         CREATE INDEX IF NOT EXISTS idx_secrets_namespace ON secrets(namespace);
         CREATE INDEX IF NOT EXISTS idx_secrets_kind ON secrets(kind);
         CREATE INDEX IF NOT EXISTS idx_secrets_tags ON secrets USING GIN(tags);
         CREATE INDEX IF NOT EXISTS idx_secrets_metadata ON secrets USING GIN(metadata jsonb_path_ops);
 
+        -- Key-value config table: stores Argon2id salt (shared across devices)
+        CREATE TABLE IF NOT EXISTS kv_config (
+            key   TEXT PRIMARY KEY,
+            value BYTEA NOT NULL
+        );
+
         CREATE TABLE IF NOT EXISTS audit_log (
             id          BIGINT GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
             action      VARCHAR(32)  NOT NULL,
@@ -60,3 +86,25 @@ pub async fn migrate(pool: &PgPool) -> Result<()> {
     tracing::debug!("migrations complete");
     Ok(())
 }
+
+/// Load the Argon2id salt from the database.
+/// Returns None if not yet initialized.
+pub async fn load_argon2_salt(pool: &PgPool) -> Result<Option<Vec<u8>>> {
+    let row: Option<(Vec<u8>,)> =
+        sqlx::query_as("SELECT value FROM kv_config WHERE key = 'argon2_salt'")
+            .fetch_optional(pool)
+            .await?;
+    Ok(row.map(|(v,)| v))
+}
+
+/// Store the Argon2id salt in the database (only called once on first device init).
+pub async fn store_argon2_salt(pool: &PgPool, salt: &[u8]) -> Result<()> {
+    sqlx::query(
+        "INSERT INTO kv_config (key, value) VALUES ('argon2_salt', $1) \
+         ON CONFLICT (key) DO NOTHING",
+    )
+    .bind(salt)
+    .execute(pool)
+    .await?;
+    Ok(())
+}