feat: Phase 1 Group 9 — enforce NOT NULL on all account_id columns

All previously-nullable account_id columns are now NOT NULL.
tree_embeddings and feedback backfilled before constraint applied.
Global content assigned to platform sentinel account (00000000-...-0001)
in preceding migration.

Tables updated: users, trees, tree_categories, tree_tags,
step_categories, step_library, tree_embeddings, feedback

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

backend/app/models/user.py

@@ -43,10 +43,10 @@ class User(Base):
     must_change_password: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False, server_default="false")
 
     # Account-based multi-tenancy (new)
-    account_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+    account_id: Mapped[uuid.UUID] = mapped_column(
         UUID(as_uuid=True),
         ForeignKey("accounts.id", ondelete="RESTRICT"),
-        nullable=True,
+        nullable=False,
         index=True
     )
     account_role: Mapped[str] = mapped_column(String(50), nullable=False, default="engineer")