feat: flexible intake — deferred variables + prepared sessions

Remove blocking intake form modal. Variables are now filled inline during
flow execution or pre-filled via prepared sessions. Adds PATCH /sessions/{id}/variables
endpoint, POST /sessions/prepare for session pre-staging, inline variable prompts
in StepDetail, editable Session Variables panel, and "Prepared for You" dashboard section.

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

backend/alembic/versions/053_add_prepared_session_fields.py

@@ -0,0 +1,40 @@
+"""add prepared session fields
+
+Revision ID: 053
+Revises: 052
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import UUID
+
+revision = "053"
+down_revision = "052"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # Add prepared_by_id and assigned_to_id for prepared sessions
+    op.add_column("sessions", sa.Column("prepared_by_id", UUID(as_uuid=True), nullable=True))
+    op.add_column("sessions", sa.Column("assigned_to_id", UUID(as_uuid=True), nullable=True))
+
+    # Make started_at nullable (prepared sessions have started_at = NULL)
+    op.alter_column("sessions", "started_at", existing_type=sa.DateTime(timezone=True), nullable=True)
+
+    # Add foreign key constraints
+    op.create_foreign_key("fk_sessions_prepared_by_id", "sessions", "users", ["prepared_by_id"], ["id"])
+    op.create_foreign_key("fk_sessions_assigned_to_id", "sessions", "users", ["assigned_to_id"], ["id"])
+
+    # Add indexes for efficient querying
+    op.create_index("ix_sessions_prepared_by_id", "sessions", ["prepared_by_id"])
+    op.create_index("ix_sessions_assigned_to_id", "sessions", ["assigned_to_id"])
+
+
+def downgrade():
+    op.drop_index("ix_sessions_assigned_to_id", table_name="sessions")
+    op.drop_index("ix_sessions_prepared_by_id", table_name="sessions")
+    op.drop_constraint("fk_sessions_assigned_to_id", "sessions", type_="foreignkey")
+    op.drop_constraint("fk_sessions_prepared_by_id", "sessions", type_="foreignkey")
+    op.alter_column("sessions", "started_at", existing_type=sa.DateTime(timezone=True), nullable=False)
+    op.drop_column("sessions", "assigned_to_id")
+    op.drop_column("sessions", "prepared_by_id")