feat: add account-based subscription model with migrations

Transition from team-based to account-based multi-tenancy (Free/Pro/Team).
Migrations 016-020 create accounts, subscriptions, plan_limits, and
account_invites tables, then migrate existing users and content FKs.

New models: Account, Subscription, PlanLimits, AccountInvite.
Updated models add account_id alongside existing team_id (coexistence
for safe two-PR deployment). Permissions and deps refactored for
account_role instead of is_team_admin.

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

backend/app/models/step_category.py

@@ -8,6 +8,7 @@ from app.core.database import Base
 
 if TYPE_CHECKING:
     from app.models.team import Team
+    from app.models.account import Account
     from app.models.user import User
 
 
@@ -37,6 +38,12 @@ class StepCategory(Base):
         nullable=True,
         index=True
     )
+    account_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=True,
+        index=True
+    )
     display_order: Mapped[int] = mapped_column(Integer, nullable=False, default=0, index=True)
     is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
     created_by: Mapped[Optional[uuid.UUID]] = mapped_column(
@@ -56,9 +63,10 @@ class StepCategory(Base):
 
     # Relationships
     team: Mapped[Optional["Team"]] = relationship("Team", back_populates="step_categories")
+    account: Mapped[Optional["Account"]] = relationship("Account", foreign_keys=[account_id], back_populates="step_categories")
     creator: Mapped[Optional["User"]] = relationship("User", foreign_keys=[created_by])
 
     @property
     def is_global(self) -> bool:
-        """Returns True if this is a global category (not team-specific)."""
-        return self.team_id is None
+        """Returns True if this is a global category (not team or account-specific)."""
+        return self.team_id is None and self.account_id is None