fix: close race conditions in script builder session and slug creation

- script_builder endpoint: pg_advisory_xact_lock on user_id before
  session count check, preventing concurrent creates from both passing
  the MAX_SESSIONS_PER_USER guard
- script_builder_service send_message: pg_advisory_xact_lock on session_id
  before message count check, preventing concurrent sends from both
  passing the MAX_MESSAGES_PER_SESSION guard
- script_builder_service save_to_library: replace check-then-insert slug
  logic with IntegrityError retry loop (3 attempts with fresh UUID suffix);
  add unique constraint on script_templates.slug (migration 070)
- ScriptBuilderPage: add creatingSessionRef to serialize concurrent
  handleSend calls that would otherwise both call createSession() while
  session is still null

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

backend/alembic/versions/070_add_unique_constraint_script_template_slug.py

@@ -0,0 +1,29 @@
+"""add unique constraint to script_templates.slug
+
+Revision ID: 070
+Revises: 069
+Create Date: 2026-04-01
+"""
+from alembic import op
+
+
+revision = "070"
+down_revision = "069"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_unique_constraint(
+        "uq_script_templates_slug",
+        "script_templates",
+        ["slug"],
+    )
+
+
+def downgrade() -> None:
+    op.drop_constraint(
+        "uq_script_templates_slug",
+        "script_templates",
+        type_="unique",
+    )