feat(agent): add voice input and photo attachments to assistant

Wire mic through Whisper-compatible transcriptions on LLM_BASE_URL.

Photos upload to MinIO and reach the vision model as base64 image_url parts.

src/app/api/agent/transcribe/route.ts

@@ -0,0 +1,51 @@
+import { apiError, apiJson } from "@/lib/api-handler";
+import { resolveApiAuth } from "@/lib/api-auth";
+import { getAssistantPreferences } from "@/lib/assistant-preference";
+import { transcribeAudioFile } from "@/lib/llm/transcribe";
+
+export const runtime = "nodejs";
+export const maxDuration = 120;
+
+const MAX_AUDIO_BYTES = 25 * 1024 * 1024;
+
+export async function POST(request: Request) {
+  const auth = await resolveApiAuth(request);
+  if (!auth?.userId) {
+    return apiError("Unauthorized", 401);
+  }
+
+  const assistant = await getAssistantPreferences(auth.userId);
+  if (!assistant.enabled) {
+    return apiError("Assistant not enabled", 403);
+  }
+
+  let formData: FormData;
+  try {
+    formData = await request.formData();
+  } catch {
+    return apiError("Invalid multipart body", 400);
+  }
+
+  const file = formData.get("file");
+  if (!(file instanceof File)) {
+    return apiError("No audio file in request", 400);
+  }
+
+  if (!file.type.startsWith("audio/") && file.type !== "video/webm") {
+    return apiError("Only audio recordings are allowed", 415);
+  }
+
+  if (file.size > MAX_AUDIO_BYTES) {
+    return apiError("Recording exceeds 25 MB limit", 413);
+  }
+
+  const filename = file.name.trim() || "recording.wav";
+
+  try {
+    const text = await transcribeAudioFile(file, filename);
+    return apiJson({ text });
+  } catch (err) {
+    const message = err instanceof Error ? err.message : "Transcription failed";
+    return apiError(message, 502);
+  }
+}