Harden backup against GitLab soft-deletion and mirror branch protection

[SupremeCommanderHedgehog]

Fixes the silent backup failures tracked in #2.

Root causes

1. GitLab soft-deletion (outage, all repos). A project deleted on gitlab.com Free lingers ~7 days in read-only retention, still occupying its path. git push --mirror → 403 You are not allowed to push code to this project. Get-GitLabProject sees it as existing (HTTP 200, not 404), so the script neither recreates nor restores it — it 403-loops every run.
2. Default-branch protection (chronic, active repos). GitLab auto-protects the default branch; push --mirror needs force-updates on active repos (rebased branches, deleted PR refs), which the protected-branch hook rejects. Private repos only needed fast-forwards, masking this for weeks.

Changes

• Get-GitLabProjectDeletionSchedule — reads marked_for_deletion_at (fallback to deprecated marked_for_deletion_on) via PSObject, so an absent field yields $null under Set-StrictMode instead of throwing.
• Restore-GitLabProject — restores a pending-deletion project (numeric-id route; the encoded-path form returns 405).
• Clear-GitLabProtectedBranches — strips mirror branch protection before each push (per_page=100; empty-list result filtered for StrictMode).
• backup.ps1 loop wires these in and logs a WARN on restore.

Verification

• Three independent full backup.ps1 runs; final: 7 succeeded, 0 failed, exit 0.
• PSScriptAnalyzer clean on both files.
• High-effort code review: 2 confirmed correctness findings applied (StrictMode field access; pagination); 2 plausible cleanups consciously dismissed (async-restore race — empirically synchronous; per-run strip cost — negligible, creation-time alternative uses a deprecated field).

🤖 Generated with Claude Code

[SupremeCommanderHedgehog]

Squash-merged to main as 2d7e9eb, signed with the repo owner's GPG key (5741F291946EBD4A8B698BA1BE707B220C995478). Done locally rather than via the GitHub merge button because gh pr merge produces an unsigned squash commit. Closes #2.