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/billing/models.py

@@ -0,0 +1,25 @@
+from datetime import datetime
+from decimal import Decimal
+from sqlalchemy import String, Boolean, DateTime, Text, Numeric, Integer, UniqueConstraint, Index
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from app.database import Base
+
+
+class PricingTier(Base):
+    __tablename__ = "pricing_tiers"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    category_key: Mapped[str] = mapped_column(String(100), nullable=False)
+    quality_level: Mapped[str] = mapped_column(String(50), nullable=False, default="Normal")
+    price_per_item: Mapped[Decimal] = mapped_column(Numeric(10, 2), nullable=False)
+    description: Mapped[str | None] = mapped_column(Text, nullable=True)
+    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)
+
+    output_types: Mapped[list["OutputType"]] = relationship("OutputType", back_populates="pricing_tier")
+
+    __table_args__ = (
+        UniqueConstraint("category_key", "quality_level", name="uq_pricing_tier"),
+        Index("ix_pricing_tiers_category_key", "category_key"),
+    )