feat: Phase 1 Group 10 — create global content tables and platform account

Creates template_trees and platform_steps (no account_id, no RLS).
Migrates is_default=TRUE trees and public steps into them.
Creates sentinel platform account (00000000-...-0001) for global
tree_categories, tree_tags, step_categories, step_library, and
is_default trees — clearing all NULL account_id rows in those tables
as prerequisite for Group 9 SET NOT NULL.

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

backend/app/models/platform_step.py

@@ -0,0 +1,37 @@
+"""Platform step model — platform-owned steps, readable by all users.
+
+No account_id. No RLS. Readable by any authenticated user.
+Populated by promoting visibility='public' steps from step_library.
+"""
+import uuid
+from datetime import datetime, timezone
+from typing import Optional, Any
+
+from sqlalchemy import String, Boolean, DateTime, ForeignKey
+from sqlalchemy.orm import Mapped, mapped_column
+from sqlalchemy.dialects.postgresql import UUID, JSONB
+
+from app.core.database import Base
+
+
+class PlatformStep(Base):
+    __tablename__ = "platform_steps"
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    title: Mapped[str] = mapped_column(String(255), nullable=False)
+    step_type: Mapped[str] = mapped_column(String(50), nullable=False, index=True)
+    content: Mapped[dict[str, Any]] = mapped_column(JSONB, nullable=False)
+    is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
+    source_step_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("step_library.id", ondelete="SET NULL"),
+        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),
+    )