refactor(arr): compute translator billing per call

Backend::characters_billed() polled a cumulative counter that races
under concurrent closes and can't attribute cost to one call. Drop it
in favour of the caller counting source characters before it sends
anything, which #197's budget needs anyway.

crates/arr-subs/src/deepl.rs

@@ -10,13 +10,13 @@
 //! One request translates a whole batch: the texts go out as a JSON array
 //! and come back as a same-length array of translations, order preserved,
 //! so cue numbering never leaves [`crate::translate`]. Characters sent are
-//! what `DeepL` bills, so they are counted and reported for #197's budget.
+//! what `DeepL` bills; #197's budget counts those itself before this ever
+//! runs, rather than polling this client for them afterwards.
 //!
 //! The auth key arrives from bootstrap config or environment per §10 —
 //! `ARR_TRANSLATE_DEEPL_API_KEY`, plumbed through `arr-daemon`'s config —
 //! never a database row.
 
-use std::sync::atomic::{AtomicU64, Ordering};
 use std::time::Duration;
 
 use reqwest::{Client, StatusCode, Url};
@@ -64,7 +64,6 @@ pub struct DeepL {
     base_url: Url,
     config: DeepLConfig,
     id: BackendId,
-    billed: AtomicU64,
 }
 
 // Hand-written: the key must never reach a log line.
@@ -109,7 +108,6 @@ impl DeepL {
             base_url: url,
             config,
             id: BackendId::new("deepl"),
-            billed: AtomicU64::new(0),
         })
     }
 
@@ -186,10 +184,6 @@ impl Backend for DeepL {
         deep_code(target, false).is_some()
     }
 
-    fn characters_billed(&self) -> u64 {
-        self.billed.load(Ordering::Relaxed)
-    }
-
     fn translate<'a>(&'a self, batch: &'a Batch) -> TranslateFuture<'a> {
         Box::pin(async move {
             let texts: Vec<String> = batch.cues.iter().map(|cue| cue.text.clone()).collect();
@@ -277,11 +271,6 @@ impl Backend for DeepL {
                 });
             }
 
-            self.billed.fetch_add(
-                texts.iter().map(|text| text.chars().count() as u64).sum(),
-                Ordering::Relaxed,
-            );
-
             Ok(parsed
                 .translations
                 .into_iter()
@@ -455,25 +444,6 @@ mod tests {
         assert_eq!(body["source_lang"], "PT");
     }
 
-    #[tokio::test]
-    async fn characters_sent_are_reported_as_billed() {
-        let server = MockServer::start().await;
-        mount_translate(&server, 200, &reply_body(&[r#""x""#])).await;
-        let client = deepl(server.uri());
-        assert_eq!(client.characters_billed(), 0);
-
-        client
-            .translate(&batch(
-                Language::Other("en".to_owned()),
-                Language::PortugueseBrazil,
-                &["four"],
-            ))
-            .await
-            .expect("the mock answers");
-
-        assert_eq!(client.characters_billed(), 4);
-    }
-
     #[tokio::test]
     async fn a_wrong_translation_count_is_a_mismatch_not_a_subtitle() {
         let server = MockServer::start().await;

crates/arr-subs/src/google.rs

@@ -13,15 +13,14 @@
 //!
 //! * replies HTML-escape characters even with `format: "text"`
 //!   (`"`, `'`), so they are decoded before returning;
-//! * per-character usage is not echoed back, so characters sent are counted
-//!   on this side and reported for #197's budget.
+//! * per-character usage is not echoed back. #197's budget does not need it
+//!   from here either — it counts source characters itself before this ever
+//!   runs.
 //!
 //! The API key arrives from bootstrap config or environment per §10 —
 //! `ARR_TRANSLATE_GOOGLE_API_KEY`, plumbed through `arr-daemon`'s config —
 //! never a database row.
 
-use std::sync::atomic::{AtomicU64, Ordering};
-
 use reqwest::{Client, StatusCode, Url};
 use serde::{Deserialize, Serialize};
 
@@ -58,7 +57,6 @@ pub struct Google {
     base_url: Url,
     config: GoogleConfig,
     id: BackendId,
-    billed: AtomicU64,
 }
 
 // Hand-written: the key must never reach a log line.
@@ -97,7 +95,6 @@ impl Google {
             base_url: url,
             config,
             id: BackendId::new("google"),
-            billed: AtomicU64::new(0),
         })
     }
 }
@@ -200,10 +197,6 @@ impl Backend for Google {
         google_code(target).is_some()
     }
 
-    fn characters_billed(&self) -> u64 {
-        self.billed.load(Ordering::Relaxed)
-    }
-
     fn translate<'a>(&'a self, batch: &'a Batch) -> TranslateFuture<'a> {
         Box::pin(async move {
             let texts: Vec<String> = batch.cues.iter().map(|cue| cue.text.clone()).collect();
@@ -285,11 +278,6 @@ impl Backend for Google {
                 });
             }
 
-            self.billed.fetch_add(
-                texts.iter().map(|text| text.chars().count() as u64).sum(),
-                Ordering::Relaxed,
-            );
-
             Ok(parsed
                 .data
                 .translations
@@ -523,25 +511,6 @@ mod tests {
         assert_eq!(body["source"], "pt");
     }
 
-    #[tokio::test]
-    async fn characters_sent_are_reported_as_billed() {
-        let server = MockServer::start().await;
-        mount_translate(&server, 200, &reply_body(&[r#""x""#])).await;
-        let client = google(server.uri());
-        assert_eq!(client.characters_billed(), 0);
-
-        client
-            .translate(&batch(
-                Language::Other("en".to_owned()),
-                Language::PortugueseBrazil,
-                &["four"],
-            ))
-            .await
-            .expect("the mock answers");
-
-        assert_eq!(client.characters_billed(), 4);
-    }
-
     #[tokio::test]
     async fn a_wrong_translation_count_is_a_mismatch_not_a_subtitle() {
         let server = MockServer::start().await;

crates/arr-subs/src/translate.rs

@@ -114,6 +114,13 @@ pub type TranslateFuture<'a> =
 /// A backend translates one batch and reports what happened. It does not
 /// chunk, does not validate its own replies — [`translate`] does both, so a
 /// backend cannot skip them — and never sees a timestamp.
+///
+/// It also does not report what it billed. §15's translator budget (#197)
+/// spends against source characters sent, which the caller already knows
+/// before any batch goes out — a cumulative counter polled off the backend
+/// cannot attribute one call's cost when several run concurrently on the
+/// same instance (§8 runs up to four closes at once), so nothing here tracks
+/// it.
 pub trait Backend: fmt::Debug + Send + Sync {
     /// The name this backend answers to in settings and the UI.
     fn id(&self) -> BackendId;
@@ -128,15 +135,6 @@ pub trait Backend: fmt::Debug + Send + Sync {
         BATCH_CHAR_BUDGET
     }
 
-    /// Characters this backend has billed so far, cumulative over its life.
-    ///
-    /// Translators charge per character (DESIGN.md §15); the daily budget
-    /// (#197) spends against what was actually billed rather than an
-    /// estimate of it. A backend that cannot say reports zero.
-    fn characters_billed(&self) -> u64 {
-        0
-    }
-
     /// Translate one batch.
     ///
     /// The reply is expected to carry every cue number the batch carried,