feat: add branch support to CDN content delivery configs

[dkrizan]

Summary

• Links CDN configs to branches via a foreign key (branch_id) instead of the plain filterBranch string column
• Adds branch selector to the CDN config dialog when branching is enabled
• Shows branch name chip on CDN list items
• Cleans up CDN configs when a branch is deleted
• Migrates existing configs to the default branch during branching enablement
• Feature guard rejects CDN create/update with filterBranch when BRANCHING feature is disabled

Test plan

• All 12 ContentDeliveryConfigBranchingTest tests pass, including:
  • Create/update CDN config with specific branch
  • Create without branch defaults to default branch
  • Non-existent branch returns 404
  • Delete branch cascades to CDN configs
  • Feature guard rejects create when branching not enabled
  • Feature guard rejects update when branching not enabled

🤖 Generated with Claude Code

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch dkrizan/cdn-branch-support

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