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fix(user_reports): drop and recreate materialized view around djstripe migrations DEV-2015 (#6960)

### 👷 Description for instance maintainers
This PR resolves a migration failure caused by the
`user_reports_userreportsmv` materialized view, which depends on columns
altered by djstripe 2.9 migrations (notably `djstripe_price.type` and
`djstripe_product.type` in `djstripe.0013_2_9`).

Without this fix, `djstripe.0013_2_9` fails with:
`cannot alter type of a column used by a view or rule`


### Notes 

**Race-condition safeguards during DROP**
The Celery task `refresh_user_report_snapshots` periodically runs
`REFRESH MATERIALIZED VIEW CONCURRENTLY`, which would block the `DROP`.
The migration now:

1. Acquires the Redis lock `billing_and_usage_snapshot:run_lock`
(blocking, 5s timeout) to prevent new task runs.
2. If the lock is already held, calls `pg_terminate_backend()` on any
backend refreshing the MV or holding a lock on it. The terminated Celery
task then releases the Redis lock through its own `finally` block. The
MV is about to be dropped, so losing an in-flight refresh is harmless.
3. Sets a PostgreSQL `lock_timeout = '30s'` as a belt-and-braces safety
net before issuing the `DROP`.

### 👀 Preview steps

1. ℹ️ Have Stripe enabled and an account with a paid subscription plan.
2. Verify `/api/v2/user_reports/` lists users with their Stripe plans.
3. Roll djstripe back: `./manage.py migrate djstripe 0012_2_8`.
4. 🔴 [on `release/2.026.12`] Re-running migrations fails on
`djstripe.0013_2_9` with the materialized view dependency error.
5. Check out this PR branch and run `./manage.py migrate` 🎉.
6. 🟢 The `user_reports` migration drops the MV before djstripe runs, and
djstripe migrations complete successfully.
7. The LRM runner (Celery Beat task `execute_long_running_migrations`)
runs every 15 minutes on the quarter hour and will recreate the MV in
the background. To force it immediately, call
`kobo.apps.long_running_migrations.tasks.execute_long_runni... (continued)

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