feat(pilot): add origin + inline idempotency to script_builder_sessions

Phase 9 prep. Adds:
- origin VARCHAR(20) NOT NULL with CHECK ('standalone' | 'pilot_inline')
- invariant: pilot_inline rows must have ai_session_id
- partial unique index on (user_id, ai_session_id) WHERE origin='pilot_inline'
  — backs get-or-create idempotency for the inline Script Builder tab.

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

backend/alembic/versions/71efd2102f49_add_origin_discriminator_inline_.py

@@ -0,0 +1,70 @@
+"""add origin discriminator + inline idempotency to script_builder_sessions
+
+Adds:
+- origin VARCHAR(20) NOT NULL DEFAULT 'standalone' with CHECK enum
+- invariant: pilot_inline rows must have ai_session_id
+- partial unique index: one pilot_inline session per (user, pilot session)
+
+Revision ID: 71efd2102f49
+Revises: 6492ec8d2d5b
+Create Date: 2026-04-24 04:22:10.819809
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '71efd2102f49'
+down_revision = '6492ec8d2d5b'
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "script_builder_sessions",
+        sa.Column(
+            "origin",
+            sa.String(length=20),
+            nullable=False,
+            server_default=sa.text("'standalone'"),
+        ),
+    )
+    op.create_check_constraint(
+        "ck_script_builder_sessions_origin",
+        "script_builder_sessions",
+        "origin IN ('standalone', 'pilot_inline')",
+    )
+    op.create_check_constraint(
+        "ck_script_builder_sessions_origin_ai_session",
+        "script_builder_sessions",
+        "origin <> 'pilot_inline' OR ai_session_id IS NOT NULL",
+    )
+    op.create_index(
+        "ux_script_builder_sessions_pilot_inline",
+        "script_builder_sessions",
+        ["user_id", "ai_session_id"],
+        unique=True,
+        postgresql_where=sa.text("origin = 'pilot_inline'"),
+    )
+    # Drop the server_default — app code owns the default via model default.
+    op.alter_column("script_builder_sessions", "origin", server_default=None)
+
+
+def downgrade() -> None:
+    op.drop_index(
+        "ux_script_builder_sessions_pilot_inline",
+        table_name="script_builder_sessions",
+    )
+    op.drop_constraint(
+        "ck_script_builder_sessions_origin_ai_session",
+        "script_builder_sessions",
+        type_="check",
+    )
+    op.drop_constraint(
+        "ck_script_builder_sessions_origin",
+        "script_builder_sessions",
+        type_="check",
+    )
+    op.drop_column("script_builder_sessions", "origin")