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/migrations/20260618T130000-ai-stt-credentials.ts

@@ -0,0 +1,18 @@
+import { type Kysely } from 'kysely';
+
+export async function up(db: Kysely<any>): Promise<void> {
+  // Encrypted, STT-specific provider key. Separate from `api_key_enc`
+  // (the chat key) so the transcription model can use a different token.
+  // When NULL, the STT model falls back to `api_key_enc`.
+  await db.schema
+    .alterTable('ai_provider_credentials')
+    .addColumn('stt_api_key_enc', 'text', (col) => col)
+    .execute();
+}
+
+export async function down(db: Kysely<any>): Promise<void> {
+  await db.schema
+    .alterTable('ai_provider_credentials')
+    .dropColumn('stt_api_key_enc')
+    .execute();
+}