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.

tests/integration/portfolio.test.ts

@@ -8,6 +8,7 @@ import {
   getAdminPortfolioProjects,
   getPublishedPortfolioProjectBySlug,
   getPublishedPortfolioProjects,
+  resolvePortfolioSlug,
 } from "@/lib/portfolio";
 import { eq } from "drizzle-orm";
 
@@ -124,6 +125,44 @@ describe("published projects", () => {
   });
 });
 
+describe("resolvePortfolioSlug", () => {
+  it("resolves an active category slug to a category", async () => {
+    await createCategory({ slug: "web", isActive: true });
+    const resolved = await resolvePortfolioSlug("web");
+    expect(resolved?.kind).toBe("category");
+    expect(resolved?.kind === "category" && resolved.category.slug).toBe("web");
+  });
+
+  it("resolves a published project slug to a project", async () => {
+    const cat = await createCategory({ isActive: true });
+    await createProject({ categoryId: cat.id, slug: "my-project", isPublished: true });
+    const resolved = await resolvePortfolioSlug("my-project");
+    expect(resolved?.kind).toBe("project");
+    expect(resolved?.kind === "project" && resolved.project.slug).toBe("my-project");
+  });
+
+  it("prefers the category when a category and a project share a slug", async () => {
+    const cat = await createCategory({ slug: "shared", isActive: true });
+    await createProject({ categoryId: cat.id, slug: "shared", isPublished: true });
+    const resolved = await resolvePortfolioSlug("shared");
+    expect(resolved?.kind).toBe("category");
+  });
+
+  it("ignores an inactive category and falls back to a matching project", async () => {
+    // An inactive category named "hidden" must not shadow a published project "hidden".
+    await createCategory({ slug: "hidden", isActive: false });
+    const activeCat = await createCategory({ isActive: true });
+    await createProject({ categoryId: activeCat.id, slug: "hidden", isPublished: true });
+    const resolved = await resolvePortfolioSlug("hidden");
+    expect(resolved?.kind).toBe("project");
+    expect(resolved?.kind === "project" && resolved.project.slug).toBe("hidden");
+  });
+
+  it("returns null for an unknown slug", async () => {
+    expect(await resolvePortfolioSlug("does-not-exist")).toBeNull();
+  });
+});
+
 describe("referential integrity", () => {
   it("restricts deleting a category that still has projects", async () => {
     const cat = await createCategory();