Integrate GitBook image resizing service and remove cdn-cgi mode

[conico974]

Replace the existing cdn-cgi image resizing mode with integration for the GitBook image resizing service. Update the default mode to cf-fetch and ensure the application uses the new service for image resizing.

TODO: Create the value in 1Password, and wait for the corresponding PR in GBX to be merged

[changeset-bot]

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Latest commit: 1097f06

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[github-actions]

Summary of the deployments:

Version	URL	Status
Vercel		❌
Cloudflare	https://a0968fb6-gitbook-open-v2-preview.gitbook.workers.dev	✅

Test content

Site	2v	2c
GitBook	/url/gitbook.com/docs	https://a0968fb6-gitbook-open-v2-preview.gitbook.workers.dev/url/gitbook.com/docs
E2E	/url/gitbook.gitbook.io/test-gitbook-open	https://a0968fb6-gitbook-open-v2-preview.gitbook.workers.dev/url/gitbook.gitbook.io/test-gitbook-open

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Build	Status	Details	Updated (UTC)
customers-v2-cloudflare (Inspect)	⚠️ Changes detected (Review)	16 changed	Jun 3, 2026, 7:47 AM
v2-cloudflare (Inspect)	⚠️ Changes detected (Review)	16 changed	Jun 3, 2026, 7:52 AM