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/subscription.py

@@ -0,0 +1,39 @@
+import uuid
+from datetime import datetime, timezone
+from typing import Optional, TYPE_CHECKING
+from sqlalchemy import String, DateTime, ForeignKey, Boolean, Integer
+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.account import Account
+
+
+class Subscription(Base):
+    __tablename__ = "subscriptions"
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    account_id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), ForeignKey("accounts.id", ondelete="CASCADE"), unique=True, nullable=False)
+    stripe_subscription_id: Mapped[Optional[str]] = mapped_column(String(255), nullable=True)
+    stripe_price_id: Mapped[Optional[str]] = mapped_column(String(255), nullable=True)
+    plan: Mapped[str] = mapped_column(String(50), nullable=False, default="free")
+    billing_interval: Mapped[Optional[str]] = mapped_column(String(20), nullable=True)
+    status: Mapped[str] = mapped_column(String(50), nullable=False, default="active")
+    seat_limit: Mapped[Optional[int]] = mapped_column(Integer, nullable=True)
+    current_period_start: Mapped[Optional[datetime]] = mapped_column(DateTime(timezone=True), nullable=True)
+    current_period_end: Mapped[Optional[datetime]] = mapped_column(DateTime(timezone=True), nullable=True)
+    cancel_at_period_end: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
+    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
+    account: Mapped["Account"] = relationship("Account", back_populates="subscription")
+
+    @property
+    def is_active(self) -> bool:
+        return self.status in ("active", "trialing")
+
+    @property
+    def is_paid(self) -> bool:
+        return self.plan in ("pro", "team")