媒体公开读授权按 objectKey 反解 gameId 改 O(1) 查找

- 从 GAME_DISTRIBUTION_MEDIA_PREFIX 后的首段反解 game_id，按主键读那一行核对媒体归属
- 不再逐行扫描全部游戏及其 screenshots_json
- 补充 key 布局（非本前缀、缺 gameId、缺文件名、多级路径）的单测

server-rs/crates/spacetime-module/src/game_distribution.rs

@@ -4208,23 +4208,48 @@ fn game_distribution_game_is_publicly_visible(game: &GameDistributionGame) -> bo
         && game.deleted_at.is_none()
 }
 
+/// 从发布媒体 objectKey 里反解 game_id；布局不符（含非本前缀的 key）返回 `None`。
+///
+/// 媒体 key 由 api-server 按 `{prefix}{game_id}/{kind}-{uuid}.{ext}` 生成，这里只做字符串切分，
+/// 不信任调用方传入的任意 key。
+fn game_distribution_media_object_key_game_id(object_key: &str) -> Option<&str> {
+    let rest = object_key
+        .strip_prefix(shared_contracts::game_distribution::GAME_DISTRIBUTION_MEDIA_PREFIX)?;
+    let (game_id, file_name) = rest.split_once('/')?;
+    if game_id.is_empty() || file_name.is_empty() {
+        return None;
+    }
+    Some(game_id)
+}
+
 /// 已公开游戏的封面/截图 OSS objectKey 允许匿名读者换取签名读地址。
+///
+/// 媒体 key 自带 game_id，直接按主键读那一行再核对 key 是否属于它的封面/截图，避免逐行扫描全部游戏。
 pub(crate) fn game_distribution_media_has_public_read_grant(
     ctx: &ReducerContext,
     object_key: &str,
 ) -> bool {
-    ctx.db.game_distribution_game().iter().any(|game| {
-        if !game_distribution_game_is_publicly_visible(&game) {
-            return false;
-        }
-        if game.cover_object_key.as_deref() == Some(object_key) {
-            return true;
-        }
-        game.screenshots_json
-            .as_deref()
-            .and_then(|json| serde_json::from_str::<Vec<String>>(json).ok())
-            .is_some_and(|screenshots| screenshots.iter().any(|key| key == object_key))
-    })
+    let Some(game_id) = game_distribution_media_object_key_game_id(object_key) else {
+        return false;
+    };
+    let Some(game) = ctx
+        .db
+        .game_distribution_game()
+        .game_id()
+        .find(&game_id.to_string())
+    else {
+        return false;
+    };
+    if !game_distribution_game_is_publicly_visible(&game) {
+        return false;
+    }
+    if game.cover_object_key.as_deref() == Some(object_key) {
+        return true;
+    }
+    game.screenshots_json
+        .as_deref()
+        .and_then(|json| serde_json::from_str::<Vec<String>>(json).ok())
+        .is_some_and(|screenshots| screenshots.iter().any(|key| key == object_key))
 }
 
 fn game_distribution_version_snapshot(
@@ -4488,4 +4513,33 @@ mod tests {
         apply_game_distribution_frozen_metadata(&mut game, &priced);
         assert_eq!(game.price_mud_points, 100);
     }
+
+    #[test]
+    fn media_object_key_game_id_is_derived_without_scanning_all_games() {
+        const PREFIX: &str = shared_contracts::game_distribution::GAME_DISTRIBUTION_MEDIA_PREFIX;
+        assert_eq!(
+            game_distribution_media_object_key_game_id(&format!("{PREFIX}game_abc/cover-1.png")),
+            Some("game_abc")
+        );
+        assert_eq!(
+            game_distribution_media_object_key_game_id(&format!(
+                "{PREFIX}game_abc/screenshot-2.webp"
+            )),
+            Some("game_abc")
+        );
+        // 非本前缀、缺 gameId、缺文件名、以及多级路径都不认。
+        assert_eq!(
+            game_distribution_media_object_key_game_id("generated/assets/cover.png"),
+            None
+        );
+        assert_eq!(game_distribution_media_object_key_game_id(PREFIX), None);
+        assert_eq!(
+            game_distribution_media_object_key_game_id(&format!("{PREFIX}game_abc/")),
+            None
+        );
+        assert_eq!(
+            game_distribution_media_object_key_game_id(&format!("{PREFIX}/cover-1.png")),
+            None
+        );
+    }
 }