chore(deploy): Railway deployment support

[fclairamb]

Adds Railway as an installation target.

• web/docs/docs/installation/railway.md: dashboard steps, CLI path, Postgres switch, first login.
• deploy/railway/.railway/railway.ts: the same setup as Railway Infrastructure as Code (one service from ghcr.io/fclairamb/solidping:0.38, a volume at /data, healthcheck on /api/mgmt/health). Railway deprecated railway.toml/railway.json (they stop being read on 2026-12-01), so this uses the new format.
• CI: the docs job type-checks that file against the railway/iac SDK.
• README: link to the new page.

Two Railway-specific settings:

• RAILWAY_RUN_UID=0: Railway mounts volumes as root, and the image runs as the distroless nonroot user, which cannot write the SQLite file otherwise.
• SP_BASE_URL=https://${{RAILWAY_PUBLIC_DOMAIN}}. No port setting is needed, the server already honors PORT.

Not tested on a live Railway project. The config was type-checked and evaluated against railway 3.16.0, and the docs site builds.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.23%. Comparing base (d67afc2) to head (ce27e17).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #576      +/-   ##
==========================================
+ Coverage   83.19%   83.23%   +0.04%     
==========================================
  Files        1207     1207              
  Lines      124137   124137              
==========================================
+ Hits       103270   103326      +56     
+ Misses      20858    20802      -56     
  Partials        9        9              

see 18 files with indirect coverage changes

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