feat(ai): server-side voice dictation (STT) with mic in chat and editor

Add push-to-talk voice dictation that transcribes recorded audio on the
server via the workspace's OpenAI-compatible AI provider (Whisper /
gpt-4o-transcribe / self-hosted whisper), then inserts the text.

Backend:
- New `stt_api_key_enc` column + migration; STT creds parity with chat/
  embeddings (sttModel/sttBaseUrl/sttApiKey, write-only key, fallbacks to
  chat baseUrl/key). Both provider whitelists updated (service + repo).
- AiService.getTranscriptionModel + AiTranscriptionService.
- Gated POST /ai-chat/transcribe (dictation flag → 403, JWT + workspace
  scope + throttle, 25MB cap, MIME whitelist, never logs audio/key).
- New `settings.ai.dictation` workspace flag (DTO + service + audit).

Frontend:
- Wire up the Voice/STT settings card (model/base URL/key) and the
  Voice-dictation toggle.
- New `features/dictation`: useDictation (MediaRecorder state machine),
  MicButton, transcribe service; integrated into the chat composer and a
  new editor-toolbar dictation group, both gated by ai.dictation.

apps/server/src/database/repos/workspace/workspace.repo.ts

@@ -239,7 +239,7 @@ export class WorkspaceRepo {
     // is a real jsonb object, never a double-encoded string. The CASE self-heals
     // workspaces whose settings.ai.provider was previously corrupted into an
     // array/string.
-    const ALLOWED = ['driver', 'chatModel', 'embeddingModel', 'baseUrl', 'embeddingBaseUrl', 'systemPrompt'];
+    const ALLOWED = ['driver', 'chatModel', 'embeddingModel', 'baseUrl', 'embeddingBaseUrl', 'sttModel', 'sttBaseUrl', 'systemPrompt'];
     const entries = Object.entries(provider).filter(
       ([k, v]) => v !== undefined && ALLOWED.includes(k),
     );