fix(build): stop bundling the better-sqlite3 stub at runtime (#11343)

next.config.mjs aliased `better-sqlite3` to its build-time stub
unconditionally, recording the premise that "runtime still uses the real
package via serverExternalPackages". That premise does not hold: a
Turbopack resolveAlias rewrites the request BEFORE the externals check
runs, so the request stopped matching the serverExternalPackages entry
and the stub was baked into the shipped bundle.

Every artifact built from the release tip then answered HTTP 500 on
every route -- the sync driver failed with "r(...) is not a constructor"
(the minified stub export), fell through node:sqlite and sql.js, and the
instrumentation hook aborted at boot.

Same failure shape as #6344, one alias above it in the same object, so
it gets the same treatment: a shared flag helper makes the alias opt-in
via OMNIROUTE_BETTER_SQLITE3_STUB=1, and a default build externalizes
the real native addon. Nobody sets the flag today; it exists for a build
host that genuinely hits the SIGABRT worker teardown from #10060, and
such a build is not shippable -- which the helper and the stub header
now say explicitly instead of describing the stub as a harmless
build-only stand-in.

Closes #11343

scripts/build/better-sqlite3-stub-flag.mjs

@@ -0,0 +1,36 @@
+/**
+ * Decide whether the Next.js build should alias `better-sqlite3` to the
+ * build-time stub (src/lib/db/better-sqlite3.stub.js).
+ *
+ * History (#11343): the alias was UNCONDITIONAL, added to keep the bundler from
+ * tracing the native addon into a Next.js build worker, whose thread teardown
+ * can abort with SIGABRT (assertion in node::RemoveEnvironmentCleanupHook) and
+ * leave the build without standalone output (#10060).
+ *
+ * The premise recorded next to that alias — "runtime still uses the real
+ * package via serverExternalPackages" — does not hold. A Turbopack
+ * `resolveAlias` rewrites the request BEFORE the externals check runs, so
+ * `better-sqlite3` becomes a relative path, no longer matches the
+ * `serverExternalPackages` entry, and the stub is baked into the bundle. Every
+ * artifact built from that config answered HTTP 500 on every route: the stub's
+ * default export is not a constructor, the sync driver chain fell through to
+ * `node:sqlite` and then sql.js, and the instrumentation hook aborted at boot.
+ *
+ * This is the same failure shape as #6344 (the @/mitm/manager stub shipping to
+ * every npm/Electron/VPS artifact), so it gets the same treatment: the alias is
+ * opt-in, and a default build gets the real, externalized native package.
+ *
+ * Set OMNIROUTE_BETTER_SQLITE3_STUB=1 ONLY on a build host that actually hits
+ * the SIGABRT worker teardown, and never for an artifact that will be run —
+ * the resulting bundle cannot open a database.
+ */
+export function shouldStubBetterSqlite3(env = process.env) {
+  return env.OMNIROUTE_BETTER_SQLITE3_STUB === "1";
+}
+
+/** Turbopack resolveAlias fragment for `better-sqlite3`, derived from the env. */
+export function betterSqlite3AliasFor(env = process.env) {
+  return shouldStubBetterSqlite3(env)
+    ? { "better-sqlite3": "./src/lib/db/better-sqlite3.stub.js" }
+    : {};
+}