Enable Azure App Service Health check for the dev web app

Added a dedicated health endpoint and wired Azure App Service Health check to it, so a persistently unhealthy instance is detected and eventually replaced even on the current single-instance B1 plan.

Decisions

Enable Health check now rather than deferring it until the App Service plan scales to multiple instances. It still pings the default hostname every minute and will replace a single unhealthy instance after it stays unhealthy for an hour; the full load-balancer rerouting benefit only appears with 2+ instances, but the detection-and-replacement safety net is useful at any instance count.
Add a standalone /api/health Route Handler instead of reusing an existing page, since Health check requires a path that returns 200-299 only when the app is fully healthy and does not follow redirects - a dedicated route avoids any accidental coupling to page-level redirects or metadata.
Mark the route `export const dynamic = "force-dynamic"` so every ping gets a live response rather than a statically cached one.
Set health_check_eviction_time_in_min to 2, the minimum of the provider's 2-10 range, to remove an unhealthy instance from rotation as quickly as the platform allows.
Make health_check_path/health_check_eviction_time_in_min optional on the webapp module (default null path, eviction time only passed through when a path is set) rather than hardcoding them into the module, so a future environment can opt out without editing the module itself.

Milestones

Added src/web/app/api/health/route.ts returning { status: "ok" } with 200, or { status: "error" } with 503 on a caught failure.
Added health_check_path and health_check_eviction_time_in_min variables to the webapp module and wired them into site_config.
Set health_check_path = "/api/health" and health_check_eviction_time_in_min = 2 on the dev environment's webapp module call.

Validation

npm run build confirmed /api/health compiles and is correctly marked Dynamic (ƒ) rather than static.
Ran the built standalone server locally and confirmed GET /api/health returned 200 with { "status": "ok" }.
terraform fmt -check
terraform validate