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/tree_embedding.py

@@ -37,10 +37,10 @@ class TreeEmbedding(Base):
         ForeignKey("trees.id", ondelete="CASCADE"),
         nullable=False,
     )
-    account_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+    account_id: Mapped[uuid.UUID] = mapped_column(
         UUID(as_uuid=True),
         ForeignKey("accounts.id", ondelete="CASCADE"),
-        nullable=True,
+        nullable=False,
     )
     chunk_type: Mapped[str] = mapped_column(
         String(30),