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/app/api/system.py

@@ -1,9 +1,13 @@
-from fastapi import APIRouter
+from fastapi import APIRouter, Depends
 import psutil
 from pydantic import BaseModel
+from sqlalchemy.orm import Session as DBSession
 
 from app.core.scheduler import send_useless_tip, send_weather_laundry_notification
+from app.core.tagging import backfill_tags
 from app.core.weather_cache import get_latest as get_latest_weather
+from app.core.word_memory import get_word_pool_with_counts
+from app.db.database import get_db
 
 router = APIRouter(prefix="/api/system", tags=["system"])
 
@@ -68,3 +72,16 @@ async def test_useless_tip():
 async def get_latest_weather_data():
     """直近で取得した明日の天気データを返す（未取得ならnull）"""
     return get_latest_weather()
+
+
+@router.get("/word-pool")
+async def get_word_pool():
+    """どうでもいいtips生成に使われているワードプールを出現回数順で返す"""
+    return get_word_pool_with_counts()
+
+
+@router.post("/tags/backfill")
+async def run_tags_backfill(limit: int | None = None, db: DBSession = Depends(get_db)):
+    """過去のユーザーメッセージのうち、タグが未生成のものにまとめてタグを付ける"""
+    count = await backfill_tags(db, limit=limit)
+    return {"tagged_count": count}