refactor(ansible): consolidate duplicated backend migration sequence into a shared tasks file #10046

Discovered during

Self-review of PR #10039 (#10001/#10026) — composability pass.

Problem

The strict migration sequence (mode probe → local-PG check → pg_dumpall backup → python -m migrations.baseline → alembic -c migrations/alembic.ini upgrade head) now exists twice, near-identically (~70 lines each):

• autobot-slm-backend/ansible/setup-user-backend.yml (post_tasks)
• autobot-slm-backend/ansible/playbooks/update-all-nodes.yml (Play 2)

Differences are only parameterization: {{ backend_code_dir }}/{{ backend_user }} vs hardcoded /opt/autobot/autobot-backend/autobot, and task-name prefixes. Two copies will drift the first time one is touched (e.g. the #10045 backup fix would have to land twice).

Fix

1. Extract to autobot-slm-backend/ansible/roles/backend/tasks/migrate.yml (or a shared task file under ansible/tasks/), parameterized by backend_code_dir / backend_user.
2. Both playbooks switch to ansible.builtin.include_tasks (or include_role: tasks_from).
3. tests/migrations/test_ansible_migration_contract.py already pins the behavioral contract for both playbooks and must stay green unchanged — it resolves included task files or keeps asserting on the rendered playbooks.

Constraint

Pure consolidation — no behavioral change. The contract tests from PR #10039 are the regression net.

Related: PR #10039 (introduced the duplication consciously to keep the strict-mode change reviewable) · #10045 (first change that would otherwise need dual edits).