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/commands/init.rs

@@ -0,0 +1,62 @@
+use anyhow::{Context, Result};
+use rand::RngExt;
+use sqlx::PgPool;
+
+use crate::{crypto, db};
+
+pub async fn run(pool: &PgPool) -> Result<()> {
+    println!("Initializing secrets master key...");
+    println!();
+
+    // Read password (no echo)
+    let password =
+        rpassword::prompt_password("Enter master password: ").context("failed to read password")?;
+    if password.is_empty() {
+        anyhow::bail!("Master password must not be empty.");
+    }
+    let confirm = rpassword::prompt_password("Confirm master password: ")
+        .context("failed to read password confirmation")?;
+    if password != confirm {
+        anyhow::bail!("Passwords do not match.");
+    }
+
+    // Get or create Argon2id salt
+    let salt = match db::load_argon2_salt(pool).await? {
+        Some(existing) => {
+            println!("Found existing salt in database (not the first device).");
+            existing
+        }
+        None => {
+            println!("Generating new Argon2id salt and storing in database...");
+            let mut salt = vec![0u8; 16];
+            rand::rng().fill(&mut salt[..]);
+            db::store_argon2_salt(pool, &salt).await?;
+            salt
+        }
+    };
+
+    // Derive master key
+    print!("Deriving master key (Argon2id, this takes a moment)... ");
+    let master_key = crypto::derive_master_key(&password, &salt)?;
+    println!("done.");
+
+    // Store in OS Keychain
+    crypto::store_master_key(&master_key)?;
+
+    // Self-test: encrypt and decrypt a canary value
+    let canary = b"secrets-cli-canary";
+    let enc = crypto::encrypt(&master_key, canary)?;
+    let dec = crypto::decrypt(&master_key, &enc)?;
+    if dec != canary {
+        anyhow::bail!("Self-test failed: encryption roundtrip mismatch");
+    }
+
+    println!();
+    println!("Master key stored in OS Keychain.");
+    println!("You can now use `secrets add` / `secrets search` commands.");
+    println!();
+    println!("IMPORTANT: Remember your master password — it is not stored anywhere.");
+    println!("           On a new device, run `secrets init` with the same password.");
+
+    Ok(())
+}