docs(gacha): clarify G1-GG vs ZZ configuration to prevent client crashes (#175)

The G1-GG gacha code path (PR #150) included example data in comments
that used non-zero item_type/item_number/item_quantity on entry_type=100
rows. Users copying these values for ZZ servers caused client crashes
because the ZZ client interprets those fields as material requirements.

- Replace unclear PR #150 comment with explicit WARNING against using
  G1-GG example values for ZZ servers
- Add gacha configuration guide header to GachaDemo.sql explaining the
  3-table system, entry types, and item type codes
- Add Mega Potion example showing correct way to add custom items
- Reference Enumerations.md for the full item type list

server/migrations/seed/GachaDemo.sql

@@ -1,5 +1,32 @@
 BEGIN;
 
+-- =============================================================================
+-- Gacha Configuration Guide (G10+ / ZZ clients)
+-- =============================================================================
+--
+-- Gacha uses THREE tables:
+--   gacha_shop     — defines the gacha lottery (name, banners, type)
+--   gacha_entries   — defines roll costs (entry_type 0/1/...) and reward pool (entry_type 100)
+--   gacha_items     — defines the actual items awarded for each reward entry
+--
+-- Entry types:
+--   0, 1, 2, ...   = Roll cost tiers. item_type/item_number/item_quantity define the currency
+--                     and amount to deduct. "rolls" = how many random draws this tier gives.
+--   100             = Reward pool entry. The client draws from these when rolling.
+--                     For G10+/ZZ: item_type, item_number, item_quantity MUST be 0.
+--                     The actual reward items are defined in gacha_items (linked by entry ID).
+--                     "weight" controls draw probability; "rarity" sets the display star rating.
+--
+-- Item types in gacha_items (see Enumerations.md for the full list):
+--   0 = Legs, 1 = Head, 2 = Chest, 3 = Arms, 4 = Waist,
+--   5 = Melee weapon, 6 = Ranged weapon, 7 = Consumable item,
+--   8 = Furniture, 10 = Zenny, 17 = N Points, 19 = Gacha Koban, ...
+--
+-- WARNING: For G1–GG clients, gacha works differently — rewards are defined
+-- directly in gacha_entries and gacha_items is NOT used. Do not mix the two
+-- formats. See handlers_shop.go for the G1–GG code path.
+-- =============================================================================
+
 -- Start Normal Demo
 INSERT INTO gacha_shop (min_gr, min_hr, name, url_banner, url_feature, url_thumbnail, wide, recommended, gacha_type, hidden)
     VALUES (0, 0, 'Normal Demo',
@@ -31,6 +58,14 @@ INSERT INTO gacha_entries (gacha_id, entry_type, item_type, item_number, item_qu
     VALUES ((SELECT id FROM gacha_shop ORDER BY id DESC LIMIT 1), 100, 0, 0, 0, 10, 2, 0, 0, 0);
 INSERT INTO gacha_items (entry_id, item_type, item_id, quantity)
     VALUES ((SELECT id FROM gacha_entries ORDER BY id DESC LIMIT 1), 10, 3, 0);
+
+-- Example: adding a consumable item (Mega Potion, item_id=8) as a gacha reward.
+-- Note: for entry_type=100, item_type/item_number/item_quantity in gacha_entries
+-- MUST remain 0 for G10+/ZZ clients. The reward is defined only in gacha_items.
+INSERT INTO gacha_entries (gacha_id, entry_type, item_type, item_number, item_quantity, weight, rarity, rolls, daily_limit, frontier_points)
+    VALUES ((SELECT id FROM gacha_shop ORDER BY id DESC LIMIT 1), 100, 0, 0, 0, 50, 1, 0, 0, 0);
+INSERT INTO gacha_items (entry_id, item_type, item_id, quantity)
+    VALUES ((SELECT id FROM gacha_entries ORDER BY id DESC LIMIT 1), 7, 8, 1);
 -- End Normal Demo
 
 -- Start Step-Up Demo