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/account.py

@@ -0,0 +1,38 @@
+import uuid
+from datetime import datetime, timezone
+from typing import Optional, TYPE_CHECKING
+from sqlalchemy import String, DateTime, ForeignKey
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.dialects.postgresql import UUID
+from app.core.database import Base
+
+if TYPE_CHECKING:
+    from app.models.user import User
+    from app.models.subscription import Subscription
+    from app.models.tree import Tree
+    from app.models.category import TreeCategory
+    from app.models.tag import TreeTag
+    from app.models.step_category import StepCategory
+    from app.models.step_library import StepLibrary
+
+
+class Account(Base):
+    __tablename__ = "accounts"
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    name: Mapped[str] = mapped_column(String(255), nullable=False)
+    display_code: Mapped[str] = mapped_column(String(8), unique=True, nullable=False)
+    owner_id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), ForeignKey("users.id", ondelete="RESTRICT"), nullable=False)
+    stripe_customer_id: Mapped[Optional[str]] = mapped_column(String(255), nullable=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=lambda: datetime.now(timezone.utc))
+    updated_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=lambda: datetime.now(timezone.utc), onupdate=lambda: datetime.now(timezone.utc))
+
+    # Relationships
+    owner: Mapped["User"] = relationship("User", foreign_keys=[owner_id], back_populates="owned_account")
+    users: Mapped[list["User"]] = relationship("User", foreign_keys="[User.account_id]", back_populates="account")
+    subscription: Mapped[Optional["Subscription"]] = relationship("Subscription", back_populates="account", uselist=False)
+    trees: Mapped[list["Tree"]] = relationship("Tree", foreign_keys="[Tree.account_id]", back_populates="account")
+    categories: Mapped[list["TreeCategory"]] = relationship("TreeCategory", foreign_keys="[TreeCategory.account_id]", back_populates="account")
+    tags: Mapped[list["TreeTag"]] = relationship("TreeTag", foreign_keys="[TreeTag.account_id]", back_populates="account")
+    step_categories: Mapped[list["StepCategory"]] = relationship("StepCategory", foreign_keys="[StepCategory.account_id]", back_populates="account")
+    step_library: Mapped[list["StepLibrary"]] = relationship("StepLibrary", foreign_keys="[StepLibrary.account_id]", back_populates="account")