REFACTORED - Flatten portfolio category routes to /portfolio/[slug]

Merge the category and project routes under a single /portfolio/[slug]
segment via resolvePortfolioSlug (category wins over project on a slug
clash). Removes the /portfolio/category/... prefix from links, redirect,
and sitemap. Adds resolver integration tests.

lib/portfolio.ts

@@ -285,6 +285,30 @@ export const getPublishedPortfolioProjectBySlug = cache(async function (slug: st
   return mapProject(project);
 });
 
+export type ResolvedPortfolioSlug =
+  | { kind: "category"; category: PortfolioCategoryView }
+  | { kind: "project"; project: PortfolioProjectView }
+  | null;
+
+/**
+ * Resolves a `/portfolio/[slug]` segment to either a category or a project.
+ * Categories take precedence so `/portfolio/web` shows the category listing;
+ * a project slug only wins when no active category shares that slug.
+ */
+export async function resolvePortfolioSlug(slug: string): Promise<ResolvedPortfolioSlug> {
+  const category = await getActivePortfolioCategoryBySlug(slug);
+  if (category) {
+    return { kind: "category", category };
+  }
+
+  const project = await getPublishedPortfolioProjectBySlug(slug);
+  if (project) {
+    return { kind: "project", project };
+  }
+
+  return null;
+}
+
 export async function getAdminPortfolioProjectById(id: string) {
   const project = await db.query.portfolioProject.findFirst({
     where: eq(portfolioProject.id, id),