added adapter exports, generalized env-depending config construction

examples/astro/src/pages/api/[...api].ts

@@ -1,3 +1,4 @@
+import type { APIContext } from "astro";
 import { App } from "bknd";
 import { serve } from "bknd/adapter/astro";
 import { registerLocalMediaAdapter } from "bknd/adapter/node";
@@ -23,7 +24,7 @@ declare module "bknd/core" {
    interface DB extends Database {}
 }
 
-export const ALL = serve({
+export const ALL = serve<APIContext>({
    // we can use any libsql config, and if omitted, uses in-memory
    connection: {
       type: "libsql",