Initial LinkLog implementation

backend/app/services/auth_service.py

@@ -0,0 +1,18 @@
+from datetime import datetime, timezone
+from hashlib import sha256
+
+from backend.app.database import get_connection
+
+
+def hash_password(password: str) -> str:
+    return sha256(password.encode('utf-8')).hexdigest()
+
+
+def authenticate_user(username: str, password: str):
+    password_hash = hash_password(password)
+    with get_connection() as conn:
+        row = conn.execute(
+            'SELECT * FROM users WHERE username = ? AND password_hash = ?',
+            (username, password_hash),
+        ).fetchone()
+    return dict(row) if row else None