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.

frontend/src/api/client.ts

@@ -16,9 +16,14 @@ export interface SessionInfo {
   updated_at: string
 }
 
+export interface Source {
+  title: string
+  url: string
+}
+
 export interface SSEChunk {
-  type: 'session_id' | 'chunk' | 'done' | 'error'
-  value?: string
+  type: 'session_id' | 'chunk' | 'done' | 'error' | 'sources'
+  value?: string | Source[]
 }
 
 const BASE = ''
@@ -45,6 +50,7 @@ export interface MessageRecord {
   role: 'user' | 'assistant' | 'system'
   content: string
   created_at: string
+  tags: string[]
 }
 
 export async function fetchMessages(sessionId: string): Promise<MessageRecord[]> {
@@ -95,6 +101,7 @@ export function streamChat(
       const reader = res.body.getReader()
       const decoder = new TextDecoder()
       let buffer = ''
+      let finished = false
 
       while (true) {
         const { done, value } = await reader.read()
@@ -106,6 +113,7 @@ export function streamChat(
           if (line.startsWith('data: ')) {
             try {
               const parsed: SSEChunk = JSON.parse(line.slice(6))
+              if (parsed.type === 'done' || parsed.type === 'error') finished = true
               onChunk(parsed)
             } catch (e) {
               console.error('[SSE] JSON parse error:', e, '| raw:', line)
@@ -113,6 +121,12 @@ export function streamChat(
           }
         }
       }
+
+      // サーバーが done/error を送らずに接続を終了した場合（再起動・切断など）はストリーミング状態が
+      // 固定されたままになるので、ここで明示的にエラーとして扱い入力をロック解除する
+      if (!finished) {
+        onError(new Error('接続が予期せず終了しました。もう一度お試しください。'))
+      }
     })
     .catch((err) => {
       if (err.name !== 'AbortError') onError(err)