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/ai-chat/ai-provider-credentials.repo.ts

@@ -98,4 +98,42 @@ export class AiProviderCredentialsRepo {
       .where('driver', '=', driver)
       .execute();
   }
+
+  // Upsert the STT-specific encrypted key. If no row exists yet this inserts one
+  // with `apiKeyEnc` left null (the column is nullable). On conflict only
+  // `sttApiKeyEnc` / `updatedAt` are touched, so the chat & embedding keys are kept.
+  async upsertSttKey(
+    workspaceId: string,
+    driver: string,
+    sttApiKeyEnc: string,
+    trx?: KyselyTransaction,
+  ): Promise<AiProviderCredentials> {
+    const db = dbOrTx(this.db, trx);
+    return db
+      .insertInto('aiProviderCredentials')
+      .values({ workspaceId, driver, sttApiKeyEnc })
+      .onConflict((oc) =>
+        oc.columns(['workspaceId', 'driver']).doUpdateSet({
+          sttApiKeyEnc,
+          updatedAt: new Date(),
+        }),
+      )
+      .returningAll()
+      .executeTakeFirst();
+  }
+
+  // Clear only the STT-specific key; the chat & embedding keys are kept.
+  async clearSttKey(
+    workspaceId: string,
+    driver: string,
+    trx?: KyselyTransaction,
+  ): Promise<void> {
+    const db = dbOrTx(this.db, trx);
+    await db
+      .updateTable('aiProviderCredentials')
+      .set({ sttApiKeyEnc: null, updatedAt: new Date() })
+      .where('workspaceId', '=', workspaceId)
+      .where('driver', '=', driver)
+      .execute();
+  }
 }