web: deprecate youtube HLS, enable it only via env variable

it's now disabled by default because if we ever need HLS for youtube in the future, it'll be managed by the processing instance, not the web client. will probably be removed completely in next major release.

web/src/routes/settings/video/+page.svelte

@@ -1,4 +1,5 @@
 <script lang="ts">
+    import env from "$lib/env";
     import settings from "$lib/state/settings";
     import { t } from "$lib/i18n/translations";
 
@@ -54,20 +55,22 @@
     </Switcher>
 </SettingsCategory>
 
-<SettingsCategory
-    sectionId="youtube-hls"
-    title={$t("settings.video.youtube.hls")}
-    disabled={$settings.save.youtubeVideoCodec === "av1"}
-    beta
->
-    <SettingsToggle
-        settingContext="save"
-        settingId="youtubeHLS"
-        title={$t("settings.video.youtube.hls.title")}
-        description={$t("settings.video.youtube.hls.description")}
+{#if env.ENABLE_DEPRECATED_YOUTUBE_HLS}
+    <SettingsCategory
+        sectionId="youtube-hls"
+        title={$t("settings.video.youtube.hls")}
         disabled={$settings.save.youtubeVideoCodec === "av1"}
-    />
-</SettingsCategory>
+        beta
+    >
+        <SettingsToggle
+            settingContext="save"
+            settingId="youtubeHLS"
+            title={$t("settings.video.youtube.hls.title")}
+            description={$t("settings.video.youtube.hls.description")}
+            disabled={$settings.save.youtubeVideoCodec === "av1"}
+        />
+    </SettingsCategory>
+{/if}
 
 <SettingsCategory sectionId="h265" title={$t("settings.video.h265")}>
     <SettingsToggle