feat: add user authentication system

- User model (users table) with bcrypt password hashing
- JWT-based login API (POST /api/auth/login, GET /api/auth/me)
- Default admin account (admin/admin123) auto-created on startup
- Login page with centered dark-themed login card
- Route guards + Axios interceptors for token management
- Login page renders without sidebar (standalone layout)
- Gunicorn --preload to avoid worker DDL race condition

backend/app/services/__init__.py

@@ -1,6 +1,7 @@
 """Service layer __init__."""
 
 from app.services.agent import AgentService  # noqa: F401
+from app.services.auth import AuthService  # noqa: F401
 from app.services.task import TaskService  # noqa: F401
 from app.services.execution import ExecutionService  # noqa: F401
 from app.services.scheduler import SchedulerService  # noqa: F401
@@ -8,6 +9,7 @@ from app.services.notification import NotificationService  # noqa: F401
 
 __all__ = [
     "AgentService",
+    "AuthService",
     "TaskService",
     "ExecutionService",
     "SchedulerService",

backend/app/services/auth.py

@@ -0,0 +1,70 @@
+"""Auth service — JWT token handling + password hashing."""
+
+from datetime import datetime, timedelta, timezone
+
+from jose import JWTError, jwt
+from passlib.context import CryptContext
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.config import settings
+from app.models.user import User
+
+pwd_context = CryptContext(schemes=["bcrypt"], deprecated="auto")
+
+ALGORITHM = "HS256"
+ACCESS_TOKEN_EXPIRE_DAYS = 7
+
+
+def hash_password(password: str) -> str:
+    return pwd_context.hash(password)
+
+
+def verify_password(plain: str, hashed: str) -> bool:
+    return pwd_context.verify(plain, hashed)
+
+
+def create_access_token(user_id: int, username: str) -> str:
+    expire = datetime.now(timezone.utc) + timedelta(days=ACCESS_TOKEN_EXPIRE_DAYS)
+    payload = {"sub": str(user_id), "username": username, "exp": expire}
+    return jwt.encode(payload, settings.SECRET_KEY, algorithm=ALGORITHM)
+
+
+def decode_access_token(token: str) -> dict | None:
+    try:
+        payload = jwt.decode(token, settings.SECRET_KEY, algorithms=[ALGORITHM])
+        return payload
+    except JWTError:
+        return None
+
+
+class AuthService:
+    def __init__(self, db: AsyncSession):
+        self.db = db
+
+    async def authenticate(self, username: str, password: str) -> User | None:
+        stmt = select(User).where(User.username == username, User.is_active == True)
+        result = await self.db.execute(stmt)
+        user = result.scalar_one_or_none()
+        if user and verify_password(password, user.password_hash):
+            return user
+        return None
+
+    async def get_user_by_id(self, user_id: int) -> User | None:
+        return await self.db.get(User, user_id)
+
+    @staticmethod
+    async def ensure_default_admin(db: AsyncSession):
+        """Create default admin if no users exist."""
+        stmt = select(User).limit(1)
+        result = await db.execute(stmt)
+        if result.scalar_one_or_none() is None:
+            admin = User(
+                username="admin",
+                password_hash=hash_password("admin123"),
+                display_name="Administrator",
+                email="admin@taskpulse.local",
+            )
+            db.add(admin)
+            await db.flush()
+            await db.commit()