docs: add community Railway self-hosting option

[Lukem121]

Summary

• Adds a Railway self-hosting path to the README with a Deploy on Railway button pointing at the community template: https://railway.com/deploy/openseo
• Adds docs/SELF_HOSTING_RAILWAY.md covering the GHCR image + /app/.wrangler volume, DataForSEO key, and Image Auto Updates
• Community template is password-gated by default (Gate service + SITE_PASSWORD in front of OpenSEO; OpenSEO stays private / local_noauth behind the gate)
• Clearly labels the template as a community listing (not an official every-app product)
• Deploy links use clean URLs with no UTM/tracking parameters

Context

OpenSEO already ships a Docker self-host path (ghcr.io/every-app/open-seo). This template wraps that same image on Railway so people can one-click deploy with persistence. Earlier concern was that Railway has no Cloudflare Access equivalent, the template now puts a password gate in front by default.

Test plan

• README Self-Hosting section renders the button and links correctly
• docs/SELF_HOSTING_RAILWAY.md links resolve and describe Gate + SITE_PASSWORD
• Deploy button opens https://railway.com/deploy/openseo with no query params

[Lukem121]

Follow-up to #111. Community template is secure by default (password-gated)