PM-5787: Allow deferred screener assignment

[jmgasper]

What was broken

Design challenges could not be saved as drafts or launched unless a Screener member was assigned to the standard Screening phase up front.

Root cause

Closed-opportunity reviewer validation and the member input required state treated Screening like every other human review phase.

What was changed

• Kept the standard Screening reviewer configuration, phase, scorecard, and reviewer count required.
• Allowed only its member assignment slots to remain empty so a copilot can assign the Screener later.
• Preserved required assignments for Review, Checkpoint Screening, and other closed manual review phases.
• Matched both phase template IDs and legacy phase instance IDs.
• Updated the Challenge Editor documentation.

Any added/updated tests

• Added schema coverage for deferred standard Screening assignment and retained Checkpoint Screening validation.
• Added launch and Save as Draft coverage for a Design challenge without an assigned Screener.
• Added reviewer UI coverage confirming that only standard Screening member inputs are optional.
• PM-5787 focused tests, lint, and the production build pass.
• The full repository test command was run: 211 suites and 1,025 tests passed; 18 unrelated pre-existing suites failed because of stale approval fixtures, missing mocks, and unresolved test aliases.