ci(release): auto-deploy the site on minor and major releases

[x3ek]

Summary

Completes the deploy formalization: engine releases with a zero patch digit (1.1.0, 2.0.0) dispatch the content repo's Deploy site workflow, which ships the exact released GHCR image to Fly. Patch releases (1.0.1...) deliberately do not auto-deploy; the content repo's manual workflow button covers those and rollbacks.

Wiring

• Content repo side already live: deploy-site.yml (repository_dispatch engine-release + workflow_dispatch with a version input), FLY_API_TOKEN secret set from a scoped Fly deploy token.
• This PR adds the sender: a deploy-site job gated on release_created and endsWith(version, '.0').
• Requires a CONTENT_DISPATCH_TOKEN secret in this repo: fine-grained PAT scoped to squishmark-content with Contents read/write (repository_dispatch needs it). Until it's set, the job fails harmlessly on x.y.0 releases and the manual button still works.

Testing

The content-repo workflow was proven with a manual run deploying 1.0.0. The dispatch path gets its first live test on the next x.y.0 release.

— Claude