feat(dictation): gate streaming dictation behind a workspace toggle

Streaming (silence-cut) dictation was hardcoded on. Put it behind a per-workspace
flag settings.ai.dictationStreaming, default off, with batch dictation as the
default and fallback. Mirrors the existing settings.ai.dictation flag end to end:

- server: aiDictationStreaming on UpdateWorkspaceDto + workspace.service writes
  settings.ai.dictationStreaming via updateAiSettings (jsonb merge keeps siblings)
- client: IWorkspaceAiSettings.dictationStreaming, an optimistic "Streaming
  dictation" sub-toggle under "Voice dictation" (disabled when dictation is off)
- gate the MicButton streaming prop in the editor toolbar and chat composer on
  the flag instead of a literal true

When the flag is absent/false both call sites pass streaming=false, so the VAD
model/wasm are never fetched and behavior is unchanged. Reuses the existing STT
model and /ai-chat/transcribe — no new provider/model/endpoint fields.

Removes the backlog entry now that it is implemented.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/client/src/features/ai-chat/components/chat-input.tsx

@@ -35,6 +35,10 @@ export default function ChatInput({
   const [value, setValue] = useAtom(aiChatDraftAtom);
   const workspace = useAtomValue(workspaceAtom);
   const isDictationEnabled = workspace?.settings?.ai?.dictation === true;
+  // Streaming (silence-cut) dictation is opt-in per workspace; absent/false
+  // keeps the stable batch path.
+  const streamingDictation =
+    workspace?.settings?.ai?.dictationStreaming === true;
 
   const submit = (): void => {
     const text = value.trim();
@@ -71,7 +75,7 @@ export default function ChatInput({
       {isDictationEnabled && (
         <MicButton
           size="lg"
-          streaming
+          streaming={streamingDictation}
           disabled={isStreaming || disabled}
           onText={(text) => setValue((v) => (v ? `${v} ${text}` : text))}
         />