db: share one now_f64() instead of four private copies

handlers, queue, send and link_cache each carried the same SystemTime
helper; a single crate::db::now_f64() removes the drift risk.

crates/xmedia-bot/src/link_cache.rs

@@ -8,6 +8,7 @@
 //! [`Config::link_cache_ttl`]; a stale entry is dropped lazily on read and
 //! by the periodic prune in `main`.
 
+use crate::db::now_f64;
 use rusqlite::{Connection, params};
 use serde::{Deserialize, Serialize};
 use std::time::Duration;
@@ -163,13 +164,6 @@ impl LinkCache {
     }
 }
 
-fn now_f64() -> f64 {
-    std::time::SystemTime::now()
-        .duration_since(std::time::UNIX_EPOCH)
-        .map(|d| d.as_secs_f64())
-        .unwrap_or(0.0)
-}
-
 #[cfg(test)]
 mod tests {
     use super::*;