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/schemas/__init__.py

@@ -1,6 +1,7 @@
 """Pydantic schema registry."""
 
 from app.schemas.agent import AgentBatchRegister, AgentBatchRegisterResult, AgentCreate, AgentHeartbeat, AgentOut, AgentUpdate  # noqa: F401
+from app.schemas.auth import LoginRequest, TokenResponse, UserInfo, UserOut  # noqa: F401
 from app.schemas.execution import ExecutionOut, ExecutionReport  # noqa: F401
 from app.schemas.notification import (  # noqa: F401
     AlertAcknowledge,

backend/app/schemas/auth.py

@@ -0,0 +1,37 @@
+"""Pydantic schemas — Auth."""
+
+from datetime import datetime
+
+from pydantic import BaseModel, Field
+
+
+class LoginRequest(BaseModel):
+    username: str = Field(..., max_length=64)
+    password: str = Field(..., max_length=128)
+
+
+class UserInfo(BaseModel):
+    id: int
+    username: str
+    display_name: str
+    email: str
+
+    model_config = {"from_attributes": True}
+
+
+class TokenResponse(BaseModel):
+    access_token: str
+    token_type: str = "bearer"
+    user: UserInfo
+
+
+class UserOut(BaseModel):
+    id: int
+    username: str
+    display_name: str
+    email: str
+    is_active: bool
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = {"from_attributes": True}