Add session export/import as JSON

Sessions can now be downloaded as a JSON file containing the session
metadata and every message with its tags, and re-imported as a new
session from that same format.

GET /chat/sessions/{id}/export streams the session as a file download.
POST /chat/sessions/import validates the roles, then recreates the
session under a fresh UUID so importing never collides with an existing
session. The sidebar gets a per-row download button and an import button
that feeds a hidden file input.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/api/client.ts

@@ -63,6 +63,39 @@ export async function deleteSession(sessionId: string): Promise<void> {
   await fetch(`${BASE}/chat/sessions/${sessionId}`, { method: 'DELETE' })
 }
 
+export function exportSession(sessionId: string): void {
+  const a = document.createElement('a')
+  a.href = `${BASE}/chat/sessions/${sessionId}/export`
+  a.download = `session_${sessionId}.json`
+  document.body.appendChild(a)
+  a.click()
+  a.remove()
+}
+
+export interface ImportPayload {
+  session?: {
+    title?: string | null
+    model?: string | null
+    created_at?: string | null
+    updated_at?: string | null
+  }
+  messages: {
+    role: string
+    content: string
+    created_at?: string | null
+    tags?: string[]
+  }[]
+}
+
+export async function importSession(payload: ImportPayload): Promise<SessionInfo> {
+  const res = await fetch(`${BASE}/chat/sessions/import`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify(payload),
+  })
+  return parseJson<SessionInfo>(res, 'インポート')
+}
+
 export async function fetchLogs(params: {
   session_id?: string
   status?: 'ok' | 'error'