feat(core): score season packs per episode

A size band describes one episode (DESIGN.md §5.5), so both the target
penalty and the floor now compare a release's size divided by the number
of episodes it covers. The caller supplies the count — arr-core has no
IO. Movies and unknown counts divide by one, so movie scoring is
unchanged and pinned by test.

Closes #180

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/arr-api/src/series.rs

@@ -13,7 +13,7 @@
 //! - **Status is derived, never stored** (§4.2). Every series the API returns
 //!   carries a status computed from its episodes at request time.
 
-use std::collections::HashMap;
+use std::collections::{BTreeMap, HashMap};
 use std::time::{Duration, SystemTime, UNIX_EPOCH};
 
 use arr_core::tracking::{apply_auto_track, apply_tracked, RefreshedSeason};
@@ -1360,7 +1360,7 @@ pub async fn episode_releases(
     State(state): State<AppState>,
     Path(id): Path<i64>,
 ) -> Result<Json<Vec<Release>>, ApiError> {
-    load_episode(&state, id).await?;
+    let episode = load_episode(&state, id).await?;
     let mut releases = sqlx::query_as!(Release, r#"SELECT r.id AS "id!: i64", r.indexer_id AS "indexer_id!: i64", r.guid AS "guid!: String", r.name AS "name!: String", r.size AS "size!: i64", r.seeders, r.publish_date, r.download_url AS "download_url!: String", r.parsed AS "parsed!: serde_json::Value", r.score, r.verdict, r.rejected_rule FROM releases r JOIN episode_releases er ON er.release_id = r.id WHERE er.episode_id = ? ORDER BY CASE r.verdict WHEN 'eligible' THEN 0 WHEN 'waived' THEN 1 ELSE 2 END, r.score DESC, r.id"#, id)
         .fetch_all(pool(&state)?)
         .await?;
@@ -1372,7 +1372,8 @@ pub async fn episode_releases(
         .map_err(|error| ApiError::Database(error.to_string()))?
         .ok_or(ApiError::EpisodeNotFound)?
         .policy;
-    rescore(&mut releases, &policy)?;
+    let lengths = season_lengths(&state, episode.series_id).await?;
+    rescore(&mut releases, &policy, Some(&lengths))?;
     Ok(Json(releases))
 }
 
@@ -1407,6 +1408,34 @@ pub async fn grab_episode(
 
 /// Resolves a season by its number within one series, so the deck is
 /// addressed the way the UI shows seasons (`/series/{id}/seasons/{n}`).
+/// Per-season episode counts for one series: the divisor data behind
+/// per-episode size normalisation (`DESIGN.md` §5.5). A season with no
+/// revealed episodes counts zero, which `claimed_episode_count` treats as
+/// unknown.
+pub(crate) async fn season_lengths(
+    state: &AppState,
+    series_id: i64,
+) -> Result<BTreeMap<u32, u32>, ApiError> {
+    let rows = sqlx::query!(
+        r#"SELECT se.number AS "number!: i64", COUNT(e.id) AS "episodes!: i64"
+           FROM seasons se LEFT JOIN episodes e ON e.season_id = se.id
+           WHERE se.series_id = ?
+           GROUP BY se.number"#,
+        series_id
+    )
+    .fetch_all(pool(state)?)
+    .await?;
+    Ok(rows
+        .into_iter()
+        .map(|row| {
+            (
+                u32::try_from(row.number).unwrap_or_default(),
+                u32::try_from(row.episodes).unwrap_or_default(),
+            )
+        })
+        .collect())
+}
+
 async fn load_season_id(state: &AppState, series_id: i64, number: i64) -> Result<i64, ApiError> {
     sqlx::query_scalar!(
         r#"SELECT id AS "id!: i64" FROM seasons WHERE series_id = ? AND number = ?"#,
@@ -1484,7 +1513,8 @@ pub async fn season_releases(
         .map_err(|error| ApiError::Database(error.to_string()))?
         .ok_or(ApiError::SeasonNotFound)?
         .policy;
-    rescore(&mut releases, &policy)?;
+    let lengths = season_lengths(&state, series_id).await?;
+    rescore(&mut releases, &policy, Some(&lengths))?;
     Ok(Json(releases))
 }