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/db.rs

@@ -160,3 +160,9 @@ pub fn now_f64() -> f64 {
         .map(|d| d.as_secs_f64())
         .unwrap_or(0.0)
 }
+
+/// Unix timestamp in whole seconds. Same clock as [`now_f64`], for fields
+/// that store integer seconds (chat-state expiry, edit prompts).
+pub fn unix_now() -> i64 {
+    now_f64() as i64
+}