feat: Phase 1 Group 8 — add account_id to target_lists (keep team_id)

Zero rows in production — this is a schema-only migration in practice.
team_id kept for app code compatibility. Drop deferred to later cleanup.
Backfill: team_id → team admin user → account_id; fallback: created_by.

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

backend/app/models/target_list.py

@@ -9,6 +9,7 @@ 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
 
 
 class TargetList(Base):
@@ -21,6 +22,12 @@ class TargetList(Base):
         UUID(as_uuid=True), ForeignKey("teams.id", ondelete="CASCADE"),
         nullable=False, index=True
     )
+    account_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
     created_by: Mapped[Optional[uuid.UUID]] = mapped_column(
         UUID(as_uuid=True), ForeignKey("users.id", ondelete="SET NULL"), nullable=True
     )