Add OnelinerState::Manual variant

This commit introduces a new `Manual` variant to the `OnelinerState`
enum, allowing for manual overrides of the oneliner text. This change
affects the client and server components to handle and display this new
state.

A `OnelinerOverrideRequest` struct is also added for API requests to set
manual oneliners. New tests are included to ensure the `Manual` variant
serializes and deserializes correctly.

The `compute_oneliner_display` function in
`server/src/routes/user_web.rs` is updated to treat `Manual` states the
same as `Ready` states for display purposes. The
`cases_needing_oneliner_in` function in
`server/src/transcribe/recovery.rs` is updated to not retry cases that
are in a `Manual` state.

server/src/routes/user_web.rs

@@ -980,7 +980,9 @@ async fn compute_oneliner_display(
         // Missing state collapses to Empty ("unbenannt"): no title is
         // expected, regardless of why.
         match state {
-            Some(OnelinerState::Ready { text, .. }) => OnelinerDisplay::Ready(text),
+            Some(OnelinerState::Ready { text, .. } | OnelinerState::Manual { text, .. }) => {
+                OnelinerDisplay::Ready(text)
+            }
             Some(OnelinerState::Error { .. }) => OnelinerDisplay::Error,
             Some(OnelinerState::Empty { .. }) | None => OnelinerDisplay::Empty,
         }
@@ -989,7 +991,9 @@ async fn compute_oneliner_display(
         // between transcript-write and state-write where the worker is
         // still producing its result.
         match state {
-            Some(OnelinerState::Ready { text, .. }) => OnelinerDisplay::Ready(text),
+            Some(OnelinerState::Ready { text, .. } | OnelinerState::Manual { text, .. }) => {
+                OnelinerDisplay::Ready(text)
+            }
             Some(OnelinerState::Empty { .. }) => OnelinerDisplay::Empty,
             Some(OnelinerState::Error { .. }) => OnelinerDisplay::Error,
             None => OnelinerDisplay::Generating,