#17432 ci: run backend migrations gate on every PR again (revert #17415)

backend migrations applied is a required check on main. #17415 added a paths filter to backend-migrations.yml, so the workflow no longer runs on PRs that don't touch backend/db, the required check never reports, and those PRs can't merge (for example #17419). The gate already passes at once when no migration changed, so running it on every PR costs one quick job.

This PR reverts only the paths filter. It doesn't touch backend files, so under the current base workflow its own required check won't report either and it needs an admin merge once.

🤖 Generated with Claude Code

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Summary by cubic

Reverts the paths filter added in #17415 so the backend migrations gate runs on every PR again.

The filter stopped the workflow from running on PRs that don't touch backend/db, so the required backend migrations applied check never reported and those PRs couldn't merge. Since the gate already passes immediately when no migration changed, running it on every PR adds only one quick job.

Note: this PR doesn't touch backend files, so under the current base workflow its own required check won't report and it needs an admin merge.

Summary by CodeRabbit

• Workflow
  • Backend migration checks now run for all qualifying pull requests targeting main or feat-cmux-next, even when no migration files change.