docs(create_task): add category auto-assign guidance for shopping lists

Extends the category_id parameter docstring with an explicit instruction
to always call get_categories first and assign the most fitting category
to every shopping list item, plus a concrete German-category mapping.
Uncategorized items are harder to find in the store.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/mcp_familywall/server.py

@@ -704,8 +704,21 @@ def create_task(
             "Äpfel (5x)", "Hackfleisch (500g)", "Joghurt (Erdbeere, 2x)".
         description: Optional longer description.
         category_id: Optional category metaId from get_categories
-            (e.g. ``taskCategory/23431854_200``). Only meaningful for
-            shopping lists; ignored for TODO lists.
+            (e.g. ``taskCategory/23431854_200``).
+            For shopping lists: ALWAYS call get_categories first and assign
+            the most fitting category to each item. Never leave category_id
+            empty for shopping list tasks — uncategorized items are harder
+            to find in the store.
+            Example mapping (German category names from get_categories):
+
+            - Fleisch, Wurst, Speck, Kasseler → "Fleisch & Fisch"
+            - Obst, Gemüse, Kraut, Zwiebeln   → "Obst & Gemüse"
+            - Bier, Wein, Saft, Wasser        → "Getränke"
+            - Senf, Honig, Gewürze, Öl        → "Zutaten & Gewürze"
+            - Brot, Brötchen                  → "Brot & Gebäck"
+            - Milch, Käse, Joghurt, Eier      → "Milch & Käse"
+
+            For TODO lists: ignored.
         due_date: Optional due date in ISO 8601 format (e.g. ``"2026-04-30T18:00:00"``).
         assignee_ids: Optional list of member IDs from get_members to assign the task
             (e.g. ``["23431898"]``). Empty list assigns to nobody.