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/pyproject.toml

@@ -17,6 +17,7 @@ dependencies = [
     "apscheduler>=3.11.2",
     "httpx>=0.28.1",
     "discord-py>=2.7.1",
+    "janome>=0.5.0",
 ]
 
 [project.optional-dependencies]