feat: Phase 1 Group 7 — add account_id to script tables (keep team_id)

team_id is kept in all three tables — drop deferred until app code
is fully migrated off team_id references.

Tables: script_builder_sessions, script_templates, script_generations
Backfill: user_id/created_by → users.account_id

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

backend/app/models/script_builder_session.py

@@ -29,6 +29,12 @@ class ScriptBuilderSession(Base):
         nullable=False,
         index=True,
     )
+    account_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
     team_id: Mapped[Optional[uuid.UUID]] = mapped_column(
         UUID(as_uuid=True),
         ForeignKey("teams.id", ondelete="SET NULL"),