refactor(B1): migrate to domain-driven project structure

Move all models/schemas/services/routers into app/domains/.
Keep backward-compat shims in old locations for imports.
Preserves domains/rendering/tasks.py from Phase A.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/domains/auth/__init__.py

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+# Domain: auth

backend/app/domains/auth/models.py

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+import uuid
+from datetime import datetime
+from sqlalchemy import String, Boolean, DateTime, Enum as SAEnum
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.dialects.postgresql import UUID
+from app.database import Base
+import enum
+
+
+class UserRole(str, enum.Enum):
+    admin = "admin"
+    project_manager = "project_manager"
+    client = "client"
+
+
+class User(Base):
+    __tablename__ = "users"
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    email: Mapped[str] = mapped_column(String(255), unique=True, nullable=False, index=True)
+    password_hash: Mapped[str] = mapped_column(String(255), nullable=False)
+    full_name: Mapped[str] = mapped_column(String(255), nullable=False)
+    role: Mapped[UserRole] = mapped_column(SAEnum(UserRole), default=UserRole.client, nullable=False)
+    is_active: Mapped[bool] = mapped_column(Boolean, default=True, nullable=False)
+    created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
+    updated_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
+
+    orders: Mapped[list["Order"]] = relationship("Order", back_populates="created_by_user", foreign_keys="Order.created_by")
+    audit_logs: Mapped[list["AuditLog"]] = relationship("AuditLog", back_populates="user", foreign_keys="AuditLog.user_id")

backend/app/domains/auth/router.py

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+# Re-export from original router.
+from app.api.routers.auth import router
+__all__ = ["router"]

backend/app/domains/auth/schemas.py

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+import uuid
+from datetime import datetime
+from pydantic import BaseModel, EmailStr
+from app.domains.auth.models import UserRole
+
+
+class UserCreate(BaseModel):
+    email: EmailStr
+    password: str
+    full_name: str
+    role: UserRole = UserRole.client
+
+
+class UserUpdate(BaseModel):
+    full_name: str | None = None
+    is_active: bool | None = None
+    role: UserRole | None = None
+
+
+class UserOut(BaseModel):
+    id: uuid.UUID
+    email: str
+    full_name: str
+    role: UserRole
+    is_active: bool
+    created_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+class TokenResponse(BaseModel):
+    access_token: str
+    token_type: str = "bearer"
+    user: UserOut
+
+
+class LoginRequest(BaseModel):
+    email: EmailStr
+    password: str