Remove deprecated useAPIMiddleware and the generated API proxy middleware

[brandonroberts]

Summary

Remove the deprecated useAPIMiddleware option and the generated #ANALOG_API_MIDDLEWARE proxy middleware from @analogjs/vite-plugin-nitro. API routes should be served exclusively through Nitro's native file-based routing under src/server/routes/api.

Background

@analogjs/vite-plugin-nitro currently supports three ways of handling the /api prefix (packages/vite-plugin-nitro/src/lib/vite-plugin-nitro.ts):

1. hasAPIDir === true — a src/server/routes/api directory exists; Nitro serves those routes natively. (Recommended, and the default for every scaffold.)
2. hasAPIDir === false && useAPIMiddleware === true — the default when no api directory exists. Injects the generated #ANALOG_API_MIDDLEWARE, which matches the /api prefix and forwards the stripped request to $fetch/proxyRequest (fetchWithEvent/proxyRequest on the alpha line).
3. hasAPIDir === false && useAPIMiddleware === false — falls back to a Nitro routeRules proxy ({ '/api/**': { proxy: { to: '/**' } } }).

useAPIMiddleware is already marked @deprecated in options.ts, with guidance to use the src/server/routes/api folder instead.

Motivation

• Redundancy. Every first-party scaffold (all create-analog templates and both @analogjs/nx-plugin generators) already ships a populated src/server/routes/api directory, so path 2 is only reached by projects that removed it. Native routing (path 1) is the intended, supported approach.
• Less custom code. The generated middleware reimplements request forwarding that Nitro's native file-based routing already handles. Removing it drops custom code that duplicates framework functionality and shrinks the request-handling surface.
• Maintenance. Removing the option collapses three code paths into one and eliminates a generated virtual module along with its dev/build/prerender wiring.

Proposed change

• Remove the useAPIMiddleware option from Options (options.ts).
• Remove the apiMiddleware template (utils/renderers.ts) and the #ANALOG_API_MIDDLEWARE handler/virtual-module registration across the dev, build, and prerender paths.
• Remove the useAPIMiddleware === false routeRules fallback.
• API routes are served solely via src/server/routes/api (native Nitro file-based routing).
• Update docs and remove references to useAPIMiddleware.

Migration

Projects that relied on the middleware (no src/server/routes/api directory, useAPIMiddleware left on) move their API handlers into src/server/routes/api — i.e. adopt the default scaffold layout. This is standard Nitro file-based routing and is already how new Analog apps are generated.

Breaking change

Yes — this removes a public option and a default behavior. Target a major release (the alpha line). Include a migration note in the release/changelog.

Alternatives considered

• Keep it deprecated indefinitely. Retains custom forwarding code and a second routing path for no functional gain over native routing.
• Flip the default to false. Still ships and maintains the option and both fallback paths; doesn't remove the redundant code.