add auth system: users, login, JWT, protected routes

Domain: User entity, AuthPort/PasswordHashPort/SecretStore ports.
Adapters: auth (argon2 hashing, JWT tokens), secret-store (env-based),
config-sqlite user repository, http-api auth routes + extractors.
Application: auth_service. SPA: login page, auth client, protected router.

crates/adapters/secret-store/src/lib.rs

@@ -0,0 +1,56 @@
+use aes_gcm::{Aes256Gcm, Key, KeyInit, Nonce, aead::Aead};
+use base64::{Engine, engine::general_purpose::STANDARD as B64};
+use domain::SecretStore;
+use rand_core::{OsRng, RngCore};
+
+pub struct AesSecretStore {
+    key: Key<Aes256Gcm>,
+}
+
+impl AesSecretStore {
+    pub fn from_env() -> Result<Self, String> {
+        let hex_key = std::env::var("KFRAME_ENCRYPTION_KEY")
+            .map_err(|_| "KFRAME_ENCRYPTION_KEY env var is required".to_string())?;
+        let bytes = hex::decode(&hex_key)
+            .map_err(|e| format!("KFRAME_ENCRYPTION_KEY must be 64 hex chars: {e}"))?;
+        if bytes.len() != 32 {
+            return Err(format!(
+                "KFRAME_ENCRYPTION_KEY must be 32 bytes (64 hex chars), got {}",
+                bytes.len()
+            ));
+        }
+        let key = Key::<Aes256Gcm>::from_slice(&bytes);
+        Ok(Self { key: *key })
+    }
+}
+
+impl SecretStore for AesSecretStore {
+    fn encrypt(&self, plaintext: &str) -> String {
+        let cipher = Aes256Gcm::new(&self.key);
+        let mut nonce_bytes = [0u8; 12];
+        OsRng.fill_bytes(&mut nonce_bytes);
+        let nonce = Nonce::from_slice(&nonce_bytes);
+        let ciphertext = cipher
+            .encrypt(nonce, plaintext.as_bytes())
+            .expect("AES-GCM encryption should not fail");
+        let mut combined = nonce_bytes.to_vec();
+        combined.extend(ciphertext);
+        B64.encode(combined)
+    }
+
+    fn decrypt(&self, ciphertext: &str) -> String {
+        let combined = B64
+            .decode(ciphertext)
+            .expect("invalid base64 in encrypted field");
+        if combined.len() < 12 {
+            panic!("encrypted data too short");
+        }
+        let (nonce_bytes, ct) = combined.split_at(12);
+        let cipher = Aes256Gcm::new(&self.key);
+        let nonce = Nonce::from_slice(nonce_bytes);
+        let plaintext = cipher
+            .decrypt(nonce, ct)
+            .expect("AES-GCM decryption failed — wrong key or corrupted data");
+        String::from_utf8(plaintext).expect("decrypted data is not valid UTF-8")
+    }
+}