feat: add pokemon trading preferences and item tag inference

Introduce trading preference (Likes/Neutral) for Pokemon with trading skills
Infer possible hidden tags for items based on trading observations
Update import/export, wipe, and admin config to support trading data

backend/db/schema.sql

@@ -809,6 +809,7 @@ CREATE TABLE IF NOT EXISTS skills (
   id integer GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
   name text NOT NULL UNIQUE,
   has_item_drop boolean NOT NULL DEFAULT false,
+  has_trading boolean NOT NULL DEFAULT false,
   sort_order integer NOT NULL DEFAULT 0 CHECK (sort_order >= 0),
   created_by_user_id integer REFERENCES users(id) ON DELETE SET NULL,
   updated_by_user_id integer REFERENCES users(id) ON DELETE SET NULL,
@@ -964,6 +965,16 @@ CREATE TABLE IF NOT EXISTS item_favorite_things (
   PRIMARY KEY (item_id, favorite_thing_id)
 );
 
+CREATE TABLE IF NOT EXISTS pokemon_trading_items (
+  pokemon_id integer NOT NULL REFERENCES pokemon(id) ON DELETE CASCADE,
+  item_id integer NOT NULL REFERENCES items(id) ON DELETE CASCADE,
+  preference text NOT NULL CHECK (preference IN ('like', 'neutral')),
+  PRIMARY KEY (pokemon_id, item_id)
+);
+
+CREATE INDEX IF NOT EXISTS pokemon_trading_items_item_idx
+  ON pokemon_trading_items(item_id, preference, pokemon_id);
+
 CREATE TABLE IF NOT EXISTS pokemon_skill_item_drops (
   pokemon_id integer NOT NULL,
   skill_id integer NOT NULL,
@@ -1262,3 +1273,6 @@ CREATE INDEX IF NOT EXISTS entity_discussion_comments_ai_moderation_status_idx
 CREATE INDEX IF NOT EXISTS entity_discussion_comments_ai_moderation_language_idx
   ON entity_discussion_comments(entity_type, entity_id, ai_moderation_language_code, created_at, id)
   WHERE deleted_at IS NULL;
+
+ALTER TABLE skills
+  ADD COLUMN IF NOT EXISTS has_trading boolean NOT NULL DEFAULT false;