feat(plan): add hard-delete for categories (blocked when in use)

DELETE /plan/categories/:id now hard-deletes the row, guarded: refuses when any live entry/override uses the key (suggests hide instead) and keeps >=1 active category. Modal gains a per-row Smazat with inline confirm. Hiding stays via PATCH is_active.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/services/planCategory.service.ts

@@ -119,3 +119,47 @@ export async function deactivatePlanCategory(
   if ("error" in res) return res;
   return { data: { ok: true } };
 }
+
+/**
+ * Hard-delete a category. Blocked when the category is still used by any live
+ * (non-soft-deleted) plan entry or override — those rows would otherwise lose
+ * their label/color — in which case the caller should hide it instead. Also
+ * refuses to remove the last active category so the create form is never empty.
+ */
+export async function deletePlanCategory(
+  id: number,
+): Promise<CatResult<{ ok: true }>> {
+  const existing = await prisma.plan_categories.findUnique({ where: { id } });
+  if (!existing) return { error: "Kategorie nenalezena", status: 404 };
+
+  const [entryCount, overrideCount] = await Promise.all([
+    prisma.work_plan_entries.count({
+      where: { category: existing.key, is_deleted: false },
+    }),
+    prisma.work_plan_overrides.count({
+      where: { category: existing.key, is_deleted: false },
+    }),
+  ]);
+  if (entryCount + overrideCount > 0) {
+    return {
+      error:
+        "Kategorie je používána v plánu a nelze ji smazat. Můžete ji skrýt.",
+      status: 409,
+    };
+  }
+
+  if (existing.is_active) {
+    const activeCount = await prisma.plan_categories.count({
+      where: { is_active: true },
+    });
+    if (activeCount <= 1) {
+      return {
+        error: "Musí zůstat alespoň jedna aktivní kategorie",
+        status: 400,
+      };
+    }
+  }
+
+  await prisma.plan_categories.delete({ where: { id } });
+  return { data: { ok: true } };
+}