feat: Phase 1 Group 3 — add account_id to step_ratings and step_usage_log

Backfill from rater/user's account_id (not the step's account_id).
This is an explicit design decision — step rating data is attributed
to the account that performed the rating.

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

backend/app/models/step_library.py

@@ -143,6 +143,13 @@ class StepRating(Base):
         ForeignKey("users.id", ondelete="CASCADE"),
         nullable=False
     )
+    account_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+        comment="Account of the RATER (not the step owner).",
+    )
     rating: Mapped[Optional[int]] = mapped_column(Integer, nullable=True)
     was_helpful: Mapped[Optional[bool]] = mapped_column(Boolean, nullable=True)
     review_text: Mapped[Optional[str]] = mapped_column(String(500), nullable=True)
@@ -187,6 +194,13 @@ class StepUsageLog(Base):
         ForeignKey("users.id", ondelete="CASCADE"),
         nullable=False
     )
+    account_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+        comment="Account of the user who logged this usage.",
+    )
     session_id: Mapped[uuid.UUID] = mapped_column(
         UUID(as_uuid=True),
         ForeignKey("sessions.id", ondelete="CASCADE"),