feat(dictation): add realtime streaming STT (live dictation)

Layer an optional realtime speech-to-text path on top of the existing
batch dictation, so transcribed text appears as the user speaks.

Transport A2: browser <-> our server (Socket.IO `/ai-realtime`) <->
OpenAI Realtime (raw ws). The provider API key never leaves the server;
the upstream URL is SSRF-checked before connecting; the gateway enforces
the dictation+dictationRealtime gate, cookie-JWT auth and per-user/
per-workspace concurrency caps. Implemented against the GA (2026) OpenAI
Realtime transcription contract (session.update / audio.input.format /
server_vad), not the now-removed beta shape.

Editor UI B2: interim text is shown as a meta-only ProseMirror ghost
decoration (no Yjs/history noise); only completed segments are committed.
Chat shows interim as a dimmed tail. The mic button switches realtime vs
batch by the workspace flag; batch remains the default and fallback.

Server:
- AiRealtimeService (upstream ws proxy, normalized events, idle/max-
  duration timeouts, idempotent teardown) + parseUpstreamEvent unit tests
- AiRealtimeGateway (Socket.IO `/ai-realtime`) wired into AiChatModule
- admin-gated POST /ai-chat/realtime/test connectivity probe
- config: settings.ai.dictationRealtime + provider sttRealtimeModel/
  sttRealtimeBaseUrl (realtime key reuses sttApiKey; no new secret)

Client:
- pcm16 AudioWorklet (24kHz mono PCM16), RealtimeDictationClient,
  use-realtime-dictation hook (status/start/stop/cancel + onInterim/onFinal)
- RealtimeMicButton + dictation-interim ProseMirror decoration
- editor/chat integration + AI settings UI (toggle, model, test endpoint)

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

apps/client/src/features/editor/extensions/extensions.ts

@@ -123,6 +123,7 @@ import { countWords } from "alfaaz";
 import AutoJoiner from "@/features/editor/extensions/autojoiner.ts";
 import GlobalDragHandle from "@/features/editor/extensions/drag-handle.ts";
 import { CleanStyles } from "@/features/editor/extensions/clean-styles.ts";
+import { DictationInterim } from "@/features/editor/extensions/dictation-interim/dictation-interim.ts";
 
 const lowlight = createLowlight(common);
 lowlight.register("mermaid", plaintext);
@@ -343,6 +344,7 @@ export const mainExtensions = [
     },
   }),
   Selection,
+  DictationInterim,
   Attachment.configure({
     view: AttachmentView,
   }),