feat(pilot): script_builder_sessions.origin on SQLAlchemy model

Mirrors the DB column added in the prior migration. App-level default
is 'standalone' so existing callers of ScriptBuilderSession(...) work
without code changes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/models/script_builder_session.py

@@ -62,6 +62,16 @@ class ScriptBuilderSession(Base):
         nullable=True,
         comment="Link to FlowPilot session if launched from there",
     )
+    origin: Mapped[str] = mapped_column(
+        String(20),
+        nullable=False,
+        default="standalone",
+        comment=(
+            "Session origin — 'standalone' (from /script-builder) or "
+            "'pilot_inline' (from FlowPilot Script Builder tab). "
+            "Invariant: pilot_inline rows must have ai_session_id set."
+        ),
+    )
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
     )