Move community feed notifier state out of Gist

[ashryanbeats]

Context

The Community Feed Notifier currently stores seen-item state in a GitHub Gist. This is working, but it ties production state to an individual GitHub account and a PAT with gist access.

Goal

Move notifier state to infrastructure or repository-owned storage so the community site is not dependent on a person-owned gist.

Options to evaluate

• Cloudflare KV, R2, or D1 if we want state owned by site infrastructure.
• A dedicated repository branch or file if we want to keep state in GitHub.
• Another lightweight hosted store if it fits the deployment model better.

Requirements

• Preserve existing seen-item state or provide a migration path.
• Avoid reposting old backlog during migration.
• Keep manual workflow dispatch behavior for seeding/suppressing backlog.
• Document required secrets/configuration in README.
• Add tests around state read/write and migration behavior where practical.

Notes

This should be a follow-up after the current Vitest/shared feed reader work lands. Do not bundle it into the current implementation cleanup.