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_invite.py

@@ -0,0 +1,48 @@
+import uuid
+from datetime import datetime, timezone
+from typing import Optional, TYPE_CHECKING
+from sqlalchemy import String, DateTime, ForeignKey, CheckConstraint
+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
+    from app.models.user import User
+
+
+class AccountInvite(Base):
+    __tablename__ = "account_invites"
+    __table_args__ = (
+        CheckConstraint("role IN ('engineer', 'viewer')", name='ck_account_invites_role'),
+    )
+
+    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"), nullable=False)
+    invited_by_id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), ForeignKey("users.id"), nullable=False)
+    email: Mapped[str] = mapped_column(String(255), nullable=False)
+    code: Mapped[str] = mapped_column(String(32), unique=True, nullable=False)
+    role: Mapped[str] = mapped_column(String(50), nullable=False, default="engineer")
+    accepted_by_id: Mapped[Optional[uuid.UUID]] = mapped_column(UUID(as_uuid=True), ForeignKey("users.id"), nullable=True)
+    expires_at: Mapped[Optional[datetime]] = mapped_column(DateTime(timezone=True), nullable=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=lambda: datetime.now(timezone.utc))
+    used_at: Mapped[Optional[datetime]] = mapped_column(DateTime(timezone=True), nullable=True)
+
+    # Relationships
+    account: Mapped["Account"] = relationship("Account")
+    invited_by: Mapped["User"] = relationship("User", foreign_keys=[invited_by_id])
+    accepted_by: Mapped[Optional["User"]] = relationship("User", foreign_keys=[accepted_by_id])
+
+    @property
+    def is_used(self) -> bool:
+        return self.accepted_by_id is not None
+
+    @property
+    def is_expired(self) -> bool:
+        if self.expires_at is None:
+            return False
+        return datetime.now(timezone.utc) > self.expires_at
+
+    @property
+    def is_valid(self) -> bool:
+        return not self.is_used and not self.is_expired