CMS Phase 9 Wave 3: Release Archive tab, medium selector, Session/Mix browsers

Renames Genre tab to Release Archive with switch-free medium card group
(Enum.GetValues-driven). Adds MediumFields single dispatch + CutFields/SessionFields/
MixFields per-medium sections embedded by all five upload/edit forms. BatchUpload
enforces single-track invariant for Session/Mix. Adds CmsSessionBrowser (hero-image
upload) and CmsMixBrowser (waveform status + per-row Generate trigger).
ICmsReleaseService/CmsReleaseService wraps api/release endpoints.
Note: medium selector is forward-compat only — API write path pending.

DeepDrftManager/Components/Pages/Tracks/SessionFields.razor

@@ -0,0 +1,10 @@
+@* Session-medium fields. The hero image is resource-addressed (POST api/release/{id}/session/hero-image)
+   and therefore set after the release exists — managed per-row in the Sessions browser, not at create
+   time when no release id is yet assigned. This section states that contract so the admin knows where
+   the hero image is managed; it carries no input of its own. *@
+<MudItem xs="12">
+    <MudAlert Severity="Severity.Info" Dense="true" Variant="Variant.Outlined">
+        Sessions are single-track live releases. After upload, set the hero image from the
+        <strong>Release Archive → Sessions</strong> browser.
+    </MudAlert>
+</MudItem>