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/websearch.py

@@ -0,0 +1,56 @@
+import logging
+
+import httpx
+
+from app.core.config import TAVILY_API_KEY
+
+logger = logging.getLogger(__name__)
+
+TAVILY_SEARCH_URL = "https://api.tavily.com/search"
+
+WEB_SEARCH_TOOL = {
+    "type": "function",
+    "function": {
+        "name": "web_search",
+        "description": "最新情報・事実確認・知らない固有名詞など、自分の知識だけでは答えられない可能性があるときにWebを検索する。",
+        "parameters": {
+            "type": "object",
+            "properties": {
+                "query": {"type": "string", "description": "検索したい内容を表す検索クエリ"},
+            },
+            "required": ["query"],
+        },
+    },
+}
+
+
+async def search_web(query: str, max_results: int = 5) -> list[dict]:
+    """Tavily APIでWeb検索する。結果は [{title, url, content}, ...]。失敗時は空リスト。"""
+    if not TAVILY_API_KEY:
+        logger.warning("TAVILY_API_KEY is not set; skipping web search")
+        return []
+
+    try:
+        async with httpx.AsyncClient(timeout=15.0) as client:
+            response = await client.post(
+                TAVILY_SEARCH_URL,
+                json={
+                    "api_key": TAVILY_API_KEY,
+                    "query": query,
+                    "max_results": max_results,
+                },
+            )
+            response.raise_for_status()
+            data = response.json()
+    except Exception:
+        logger.exception("Tavily search failed for query=%r", query)
+        return []
+
+    results = []
+    for item in data.get("results", []):
+        results.append({
+            "title": item.get("title") or "",
+            "url": item.get("url") or "",
+            "content": item.get("content") or "",
+        })
+    return results