Add authentication and user profile endpoint

Argon2 password hashing, JWT session issuing/verification
(auth/security.py), register/login routes, the bearer-token
get_current_user dependency, and GET /users/me for address + balance.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/unit/test_security.py

@@ -0,0 +1,13 @@
+from app.auth import security
+
+
+def test_password_hash_roundtrip():
+    hashed = security.hash_password("s3cret!")
+    assert security.verify_password("s3cret!", hashed)
+    assert not security.verify_password("wrong", hashed)
+
+
+def test_jwt_roundtrip(monkeypatch):
+    monkeypatch.setattr(security.settings, "jwt_secret", "test-secret")
+    token = security.create_access_token(user_id=42)
+    assert security.decode_access_token(token) == 42