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>

app/auth/security.py

@@ -0,0 +1,31 @@
+from datetime import datetime, timedelta, timezone
+
+import jwt
+from argon2 import PasswordHasher
+from argon2.exceptions import VerifyMismatchError
+
+from app.config import settings
+
+_hasher = PasswordHasher()
+
+
+def hash_password(password: str) -> str:
+    return _hasher.hash(password)
+
+
+def verify_password(password: str, password_hash: str) -> bool:
+    try:
+        return _hasher.verify(password_hash, password)
+    except VerifyMismatchError:
+        return False
+
+
+def create_access_token(user_id: int) -> str:
+    expires_at = datetime.now(timezone.utc) + timedelta(minutes=settings.jwt_expire_minutes)
+    payload = {"sub": str(user_id), "exp": expires_at}
+    return jwt.encode(payload, settings.jwt_secret, algorithm=settings.jwt_algorithm)
+
+
+def decode_access_token(token: str) -> int:
+    payload = jwt.decode(token, settings.jwt_secret, algorithms=[settings.jwt_algorithm])
+    return int(payload["sub"])