feat: live IMAP sync on folder select + 60s auto-sync interval

- New imap_sync_folder function: syncs single folder quickly for live use
- New POST /mail/folders/{folder_id}/sync route
- Frontend: syncFolder API + called on every folder selection
- Auto-sync interval reduced from 300s to 60s
- Sent mail IMAP APPEND already uploads full body via msg.as_bytes()

app/plugins/builtins/mail/routes.py

@@ -632,6 +632,32 @@ async def empty_folder(
     return {"emptied_count": len(mails)}
 
 
+# ─── Sync single folder (live sync when selecting a folder) ───
+
+
+@router.post("/folders/{folder_id}/sync")
+async def sync_folder(
+    folder_id: str,
+    db: AsyncSession = Depends(get_db),
+    current_user: dict = Depends(require_permission("mail:read")),
+):
+    """Sync a single folder from IMAP server immediately."""
+    tenant_id = uuid.UUID(current_user["tenant_id"])
+    user_id = uuid.UUID(current_user["user_id"])
+    f_id = _parse_uuid(folder_id, "folder_id")
+    folder = (
+        await db.execute(
+            select(MailFolder).where(and_(MailFolder.id == f_id, MailFolder.tenant_id == tenant_id))
+        )
+    ).scalar_one_or_none()
+    if not folder:
+        raise HTTPException(404, detail={"detail": "Folder not found", "code": "not_found"})
+    account = await _get_account(db, folder.account_id, tenant_id, user_id)
+    result = await mail_services.imap_sync_folder(db, f_id, tenant_id)
+    await db.flush()
+    return result
+
+
 # ─── Attachment Upload (F-MAIL-04) ───