feat(discussion): add discussion feature for game entities

Create entity_discussion_comments table and API endpoints
Add discussion tabs to Pokemon, Item, Recipe, and Habitat detail views
Support top-level comments, single-level replies, and deletion

backend/db/schema.sql

@@ -639,3 +639,34 @@ CREATE INDEX IF NOT EXISTS wiki_edit_logs_entity_idx
 
 CREATE INDEX IF NOT EXISTS wiki_edit_logs_user_id_idx
   ON wiki_edit_logs(user_id);
+
+CREATE TABLE IF NOT EXISTS entity_discussion_comments (
+  id integer GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
+  entity_type text NOT NULL CHECK (entity_type IN ('pokemon', 'items', 'recipes', 'habitats')),
+  entity_id integer NOT NULL,
+  parent_comment_id integer REFERENCES entity_discussion_comments(id) ON DELETE CASCADE,
+  body text NOT NULL CHECK (length(body) BETWEEN 1 AND 1000),
+  created_by_user_id integer REFERENCES users(id) ON DELETE SET NULL,
+  deleted_by_user_id integer REFERENCES users(id) ON DELETE SET NULL,
+  deleted_at timestamptz,
+  created_at timestamptz NOT NULL DEFAULT now(),
+  updated_at timestamptz NOT NULL DEFAULT now()
+);
+
+ALTER TABLE entity_discussion_comments DROP CONSTRAINT IF EXISTS entity_discussion_comments_entity_type_check;
+ALTER TABLE entity_discussion_comments ADD CONSTRAINT entity_discussion_comments_entity_type_check CHECK (
+  entity_type IN ('pokemon', 'items', 'recipes', 'habitats')
+);
+
+ALTER TABLE entity_discussion_comments ADD COLUMN IF NOT EXISTS deleted_by_user_id integer REFERENCES users(id) ON DELETE SET NULL;
+ALTER TABLE entity_discussion_comments ADD COLUMN IF NOT EXISTS deleted_at timestamptz;
+ALTER TABLE entity_discussion_comments ADD COLUMN IF NOT EXISTS updated_at timestamptz NOT NULL DEFAULT now();
+
+CREATE INDEX IF NOT EXISTS entity_discussion_comments_entity_idx
+  ON entity_discussion_comments(entity_type, entity_id, created_at, id);
+
+CREATE INDEX IF NOT EXISTS entity_discussion_comments_parent_idx
+  ON entity_discussion_comments(parent_comment_id, created_at, id);
+
+CREATE INDEX IF NOT EXISTS entity_discussion_comments_user_idx
+  ON entity_discussion_comments(created_by_user_id);