Implement create custom recommendations

app/api/create-flow/methods/route.ts

@@ -0,0 +1,56 @@
+import { NextResponse, type NextRequest } from "next/server";
+import { isDatabaseConfigured } from "../../../../lib/server/env";
+import { listMethodRecommendations } from "../../../../lib/server/methodRecommendations";
+import { dbUnavailable } from "../../../../lib/server/responses";
+import {
+  SECTION_IDS,
+  type SectionId,
+  parseRequestedFacetsFromSearchParams,
+} from "../../../../lib/server/validation/methodFacetsSchemas";
+
+const SECTION_SET = new Set<string>(SECTION_IDS);
+
+/**
+ * GET /api/create-flow/methods?section=<section>[&facet.*=...]
+ *
+ * Returns slugs + per-method match scores for one of the four card-deck
+ * sections; the wizard renders by looking up the slug in the section's
+ * messages file (`useMessages().create.customRule.<section>.methods`).
+ *
+ * See `docs/guides/template-recommendation-matrix.md` §9.2 / §10.
+ */
+export async function GET(request: NextRequest) {
+  if (!isDatabaseConfigured()) {
+    return dbUnavailable();
+  }
+
+  const sectionParam = request.nextUrl.searchParams.get("section");
+  if (!sectionParam || !SECTION_SET.has(sectionParam)) {
+    return NextResponse.json(
+      {
+        error: {
+          code: "validation_error",
+          message: `Unknown section. Expected one of: ${SECTION_IDS.join(", ")}`,
+        },
+      },
+      { status: 400 },
+    );
+  }
+  const section = sectionParam as SectionId;
+
+  const facets = parseRequestedFacetsFromSearchParams(
+    request.nextUrl.searchParams,
+  );
+  const result = await listMethodRecommendations({ section, facets });
+  if (!result) {
+    // DB query failed; return empty so the wizard falls back to its messages
+    // deck in authoring order (§10).
+    return NextResponse.json({ section, methods: [] });
+  }
+
+  const methods = result.rankedSlugs.map((slug) => ({
+    slug,
+    matches: result.matchesBySlug[slug] ?? { score: 0, matchedFacets: [] },
+  }));
+  return NextResponse.json({ section, methods });
+}