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/knowledge.py

@@ -0,0 +1,37 @@
+from sqlalchemy.orm import Session as DBSession
+
+from app.core.config import KNOWLEDGE_CONTEXT_LIMIT
+from app.db.models import KnowledgeEntry, KnowledgeSource
+
+
+def save_knowledge(db: DBSession, query: str, summary: str, sources: list[dict]) -> KnowledgeEntry:
+    """Web検索で得た知見をDBに保存する。"""
+    entry = KnowledgeEntry(query=query, summary=summary)
+    db.add(entry)
+    db.flush()
+    for src in sources:
+        if not src.get("url"):
+            continue
+        db.add(KnowledgeSource(entry_id=entry.id, title=src.get("title", ""), url=src["url"]))
+    db.commit()
+    db.refresh(entry)
+    return entry
+
+
+def get_knowledge_context(db: DBSession) -> str | None:
+    """蓄積された知識を、システムプロンプトに注入できる形のテキストにして返す。"""
+    entries = (
+        db.query(KnowledgeEntry)
+        .order_by(KnowledgeEntry.created_at.desc())
+        .limit(KNOWLEDGE_CONTEXT_LIMIT)
+        .all()
+    )
+    if not entries:
+        return None
+
+    blocks = []
+    for entry in reversed(entries):
+        urls = ", ".join(s.url for s in entry.sources) or "出典不明"
+        blocks.append(f"- 「{entry.query}」について: {entry.summary}\n  (出典: {urls})")
+
+    return "以下はこれまでのWeb検索で得た知識です。関連する話題が出たら活用してください:\n\n" + "\n".join(blocks)