Migrate Railway configuration to infrastructure as code

[austin-smith]

What Changed

• Replace the three app-local Railway Config as Code files with one project-level .railway/railway.ts graph for the application services, databases, volumes, domains, networking, and variables.
• Add deterministic validation for both Railway environments, including preserved secrets, database references, deployment contracts, and same-major database image auto-updates.
• Add a standalone pnpm railway:check command and run it as an independent CI step.
• Update the Railway runbook with the reviewed pre-merge cutover, verification, and rollback procedure.

Why

Railway is moving from service-scoped Config as Code to project-level Infrastructure as Code. A single typed project graph makes the complete deployment contract reviewable and prevents the legacy and replacement configuration systems from coexisting after merge.

This pull request does not modify Railway. Follow docs/railway.md and do not merge until both Railway environments have completed the cutover and produced clean second plans.

Validation

• pnpm railway:check
• pnpm verify --concurrency=1 — 26/26 tasks successful
• pnpm --filter @webhooks-lol/web exec vitest run tests/next-config.test.ts tests/auth/server.test.ts — 10/10 tests passed
• Prettier check for all changed source, configuration, workflow, and documentation files
• git diff --check
• Autoreview — no accepted or actionable findings