refactor(send): apply ponytail audit cuts 2, 4, 6

- updated_sequence_task: clone the Task and mutate the two fields
  instead of rebuilding all 12 by hand (-22 lines; new fields no
  longer need a sync here)
- unify unix_now with db::now_f64 (unix_now() = now_f64() as i64),
  moved to db.rs next to its clock source
- classify_to_send_error takes the MediaFetchFailure label, folding
  the duplicated inline match in send_batch_via_upload (-8 lines)

crates/xmedia-bot/src/handlers/callback.rs

@@ -3,8 +3,8 @@
 
 use super::urls::enqueue_retry;
 use super::{CHAT_STORE, CONFIG, TASK_QUEUE};
+use crate::db::unix_now;
 use crate::send::{self, Task};
-use crate::state::unix_now;
 use teloxide::RequestError;
 use teloxide::prelude::*;
 use teloxide::types::{CallbackQuery, ChatId, MessageId, ParseMode};