ci: reduce low-signal Actions usage

[MilesCranmerBot]

This PR reduces low-signal Actions usage without removing the required Linux PR gate.

Changes

• Added PR concurrency cancellation to CI, Windows, macOS, docs workflows
• Narrowed Linux push trigger from all branches to master/release-*/tags
• Moved Docker CI to monthly/manual schedule
• Moved Docker Large Nightly from daily ARM64+AMD64 to monthly AMD64, with optional ARM64 via manual dispatch
• Moved Apptainer CI to monthly/manual
• Removed daily Docker deploy schedule (keeps push/tag triggers)
• Reduced conda-forge from daily to weekly
• Fixed backend-update to skip redispatch when PR already exists; only dispatches Linux/Windows/macOS
• Removed unnecessary pip caches from update_backend and validate_papers_yml

Expected savings

Dominant cost centres were Docker_Large_Nightly (28.5k min/90d, mostly ARM64 QEMU), Linux push on all branches (12.4k min), and conda-forge (2.5k min, all failing).

Required checks

Linux test job names are unchanged; branch protection is unaffected.

[MilesCranmer]

@MilesCranmerBot please address each comment

[MilesCranmer]

Also note that CI is failing so needs fix for that too

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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