feat: support analytics().sendEvent() from Pages Router API routes

[vklimontovich]

What

analytics() now accepts an optional request so analytics().sendEvent() works in both routers:

analytics(req?: NextRequest | NextApiRequest) => Promise<NextlyticsServerSide>

• App Router (server components, server actions, route handlers): call analytics() with no args — unchanged, context comes from next/headers.
• Pages Router API routes: call analytics(req). next/headers throws there, so context is built from req.headers / req.cookies / req.url.
• A NextRequest (App Router route handler) is also accepted.

Why

The server API was App-Router-only because analytics() reads from next/headers, which throws in Pages Router API routes (req/res). There was no nextlytics-native way to emit a server event from those routes — e.g. email pixel / redirect endpoints that genuinely need to be Pages Router.

How

The three context sources are unified behind a NormalizedRequest shape, then buildServerContext() consumes it (replacing the App-Router-only createServerContextFromHeaders). Standalone routes with no preceding page render (email pixels, redirects) now send without a parent page-view — parentEventId is omitted rather than failing. The "middleware not configured" hard error is preserved only for the no-arg App Router path.

Server context method is now the real request method instead of a hardcoded "GET".

Notes

• No change to anon-cookie behavior: like the existing App Router analytics(), no new anon cookie is written (existing cookies are read; GDPR-hash mode works).

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