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/state.rs

@@ -1,12 +1,13 @@
 //! Per-chat state with SQLite persistence (table `chat_state` in
 //! `data/task_queue.db`, shared with the task queue).
 
+use crate::db::unix_now;
 use parking_lot::Mutex;
 use rusqlite::params;
 use serde::{Deserialize, Serialize};
 use std::collections::HashMap;
 use std::sync::Arc;
-use std::time::{Duration, SystemTime, UNIX_EPOCH};
+use std::time::Duration;
 
 #[derive(Serialize, Deserialize, Default, Clone, Debug)]
 pub struct ChatData {
@@ -40,13 +41,6 @@ pub struct ChatStore {
     pool: Arc<crate::db::DbPool>,
 }
 
-pub fn unix_now() -> i64 {
-    SystemTime::now()
-        .duration_since(UNIX_EPOCH)
-        .map(|d| d.as_secs() as i64)
-        .unwrap_or(0)
-}
-
 impl ChatStore {
     /// Wraps the shared DB pool (schema initialized once by
     /// [`crate::db::open_store`]; the `chat_state` table lives in the merged