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_library.py

@@ -10,6 +10,7 @@ from app.core.database import Base
 if TYPE_CHECKING:
     from app.models.user import User
     from app.models.team import Team
+    from app.models.account import Account
     from app.models.step_category import StepCategory
     from app.models.session import Session
 
@@ -43,6 +44,12 @@ class StepLibrary(Base):
         ForeignKey("teams.id", ondelete="CASCADE"),
         nullable=True
     )
+    account_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=True,
+        index=True
+    )
 
     # Organization
     category_id: Mapped[Optional[uuid.UUID]] = mapped_column(
@@ -91,6 +98,7 @@ class StepLibrary(Base):
     # Relationships
     creator: Mapped["User"] = relationship("User", foreign_keys=[created_by])
     team: Mapped[Optional["Team"]] = relationship("Team")
+    account: Mapped[Optional["Account"]] = relationship("Account", foreign_keys=[account_id], back_populates="step_library")
     category: Mapped[Optional["StepCategory"]] = relationship("StepCategory")
     ratings: Mapped[list["StepRating"]] = relationship("StepRating", back_populates="step", cascade="all, delete-orphan")
     usage_logs: Mapped[list["StepUsageLog"]] = relationship("StepUsageLog", back_populates="step", cascade="all, delete-orphan")