chore: encrypt tenant NWC URL at rest and stop secret exposure in tenant APIs (#58)

Co-authored-by: userAdityaa <aditya.chaudhary1558@gmail.com>
Co-committed-by: userAdityaa <aditya.chaudhary1558@gmail.com>

backend/src/cipher.rs

@@ -0,0 +1,28 @@
+use anyhow::{Result, anyhow};
+use nostr_sdk::prelude::*;
+
+pub fn encrypt(plaintext: &str) -> Result<String> {
+    let keys = load_key()?;
+    nip44::encrypt(
+        keys.secret_key(),
+        &keys.public_key(),
+        plaintext,
+        nip44::Version::V2,
+    )
+    .map_err(|e| anyhow!("encryption failed: {e}"))
+}
+
+pub fn decrypt(ciphertext: &str) -> Result<String> {
+    let keys = load_key()?;
+    nip44::decrypt(keys.secret_key(), &keys.public_key(), ciphertext)
+        .map_err(|e| anyhow!("decryption failed: {e}"))
+}
+
+fn load_key() -> Result<Keys> {
+    let secret = std::env::var("ENCRYPTION_SECRET")
+        .map_err(|_| anyhow!("missing ENCRYPTION_SECRET environment variable"))?;
+    if secret.trim().is_empty() {
+        return Err(anyhow!("ENCRYPTION_SECRET is empty"));
+    }
+    Keys::parse(&secret).map_err(|e| anyhow!("invalid ENCRYPTION_SECRET: {e}"))
+}