Add web search knowledge base, message tagging, and word-memory tip topics

Introduces websearch.py (Tavily) + knowledge.py to ground chat responses
in search results, tagging.py to tag user and assistant messages via LLM
and pick related topics for the Discord useless-tips job, and
word_memory.py to track recent conversation nouns (surfaced in the new
WordPool.vue sidebar) used to pick tip topics close to a related tag.

backend/.env.example

@@ -16,6 +16,11 @@ AWS_BEARER_TOKEN_BEDROCK=
 # デバッグ (true にすると LiteLLM の詳細ログが出力される)
 DEBUG=false
 
+# Tavily Web検索（未設定だとweb_searchツールは何も検索せず空振りする）
+TAVILY_API_KEY=
+# 蓄積知識をシステムプロンプトに注入する際の最大件数
+KNOWLEDGE_CONTEXT_LIMIT=15
+
 # 天気予報 → Discord 通知
 DISCORD_WEBHOOK_URL=
 WEATHER_AREA_CODE=120000