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/core/discord_bot.py

@@ -12,8 +12,10 @@ from app.core.config import (
 )
 from app.core.llm import complete
 from app.core.scheduler import send_useless_tip
+from app.core.tagging import find_related_tag, generate_tags
 from app.core.weather import fetch_tomorrow_weather, judge_laundry
 from app.core.weather_cache import set_latest
+from app.core.word_memory import record_message
 
 logger = logging.getLogger(__name__)
 
@@ -61,12 +63,18 @@ async def on_message(message: discord.Message):
     if not content:
         content = "こんにちは"
 
+    record_message(content)
+
     async with message.channel.typing():
         try:
             if any(keyword in content for keyword in USELESS_TALK_KEYWORDS):
                 redirect = await _generate_useless_talk_redirect(content)
                 await message.reply(redirect)
-                await send_useless_tip()
+
+                # 振られた話題のタグを生成し、それに近しい話題からTipsのネタを作る
+                seed_tags = await generate_tags(content)
+                related_tag = find_related_tag(seed_tags)
+                await send_useless_tip(topic_override=related_tag)
                 return
 
             messages = [{"role": "system", "content": DISCORD_BOT_SYSTEM_PROMPT}]
@@ -78,6 +86,8 @@ async def on_message(message: discord.Message):
             messages.append({"role": "user", "content": content})
 
             reply = await complete(messages, model=WEATHER_NOTIFY_MODEL)
+            if reply:
+                record_message(reply)
             await message.reply(reply or "(応答が空でした)")
         except Exception:
             logger.exception("Failed to generate reply for Discord mention")