CI: Add interruptible jobs and auto-cancel for redundant pipelines

Problem

When a new commit is pushed to an MR, previous pipeline jobs keep running, wasting runner resources (especially the limited dle-test shell runners) and causing confusion with stale results.

Solution

1. Add default: interruptible: true so all jobs can be cancelled
2. Add workflow: auto_cancel: on_new_commit: interruptible to automatically cancel redundant pipelines
3. Deploy jobs remain non-interruptible

Expected Impact

• Reduced runner waste when multiple commits are pushed quickly
• Faster feedback for developers (old pipelines don't block runners)
• Cleaner pipeline history
• Better utilization of limited shell runners

Part of CI optimization effort

Related: postgresai MR !197 (merged)

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Acceptance Criteria

• default: interruptible: true is set in .gitlab-ci.yml so all jobs are cancellable by default
• workflow: auto_cancel: on_new_commit: interruptible automatically cancels running pipelines on new MR commits
• Deploy jobs remain interruptible: false
• Limited shell runners (dle-test) are freed up faster when new commits are pushed

Definition of Done

• CI configuration updated and merged
• Verified by pushing consecutive commits: old pipeline cancelled, new pipeline starts
• Shell runner utilization improved (less time running stale pipelines)
• No accidental cancellation of deploy jobs