refactor(db): use rusqlite query_row/optional instead of hand-rolled reads

pending_backlog, earliest_run_after, ChatStore::get and LinkCache::get each
hand-rolled prepare + query + rows.next() for what is a single-row read.
query_row + OptionalExtension::optional is the same statement and the same
error mapping with less scaffolding; the backlog's NULL-on-empty MIN still
goes through the count check, so a pending row with a NULL run_after is not
misread. Also fixes the rustfmt drift from the previous commit.

crates/xmedia-bot/src/link_cache.rs

@@ -9,6 +9,7 @@
 //! by the periodic prune in `main`.
 
 use crate::db::now_f64;
+use rusqlite::OptionalExtension;
 use rusqlite::params;
 use serde::{Deserialize, Serialize};
 use std::sync::Arc;
@@ -76,14 +77,16 @@ impl LinkCache {
         let result = self
             .pool
             .with_conn(move |conn| {
-                let mut stmt =
-                    conn.prepare("SELECT payload, created_at FROM link_cache WHERE url = ?1")?;
-                let mut rows = stmt.query(params![key])?;
-                let Some(row) = rows.next()? else {
+                let Some((payload, created_at)) = conn
+                    .query_row(
+                        "SELECT payload, created_at FROM link_cache WHERE url = ?1",
+                        params![key],
+                        |row| Ok((row.get::<_, String>(0)?, row.get::<_, f64>(1)?)),
+                    )
+                    .optional()?
+                else {
                     return Ok(None);
                 };
-                let payload: String = row.get(0)?;
-                let created_at: f64 = row.get(1)?;
                 if now_f64() - created_at > ttl {
                     conn.execute("DELETE FROM link_cache WHERE url = ?1", params![key])?;
                     return Ok(None);