fix(gateway): mediaAnalysis ffprobe path, fallback error-log, generic closer sanitize

Audit log produksi (sejak deploy13:38) menemukan 3 isu:
1. mediaDownloader.ts spawn /usr/bin/ffprobe + /usr/bin/ffmpeg (path keras) —
   ENOENT di Nix karena binary cuma di ffmpeg-headless closure. Pakai
   PATH-resolved ('ffprobe'/'ffmpeg') seperti voice-recording module
   (ffmpegProcess.ts/transmitter.ts) — 5 media warning hilang.
2. individualFallbackProcessor log error 'Success' di level50 tiap fallback
   BERHASIL (logModerationError dengan new Error('Success')) — ganti
   logger.info dengan verdict yang sama; error log cuma untuk error asli.
3. moderationResponseParser: strip frasa penutup generik ('Tidak ada
   indikasi pelanggaran.') yang masih sering dikeluarkan LLM walau prompt
   melarang (277/1486 analisis mengandung frasa, termasuk hari ini).
   sanitizeGenericCleanCloser hanya mencocok frasa di AKHIR, teks substantif
   tetap utuh. Unit test: 6/6 pass.

services/discord-gateway/src/modules/ai-moderation/mediaDownloader.ts

@@ -198,7 +198,7 @@ async function extractVideoFrames(
   try {
     await writeFile(inputPath, videoBytes);
     const { stdout: durationStr } = await execFileAsync(
-      "/usr/bin/ffprobe",
+      "ffprobe",
       [
         "-v",
         "error",
@@ -213,7 +213,7 @@ async function extractVideoFrames(
     const duration = parseFloat(durationStr.trim()) || 1;
     const fps = (3 / duration).toFixed(6);
     await execFileAsync(
-      "/usr/bin/ffmpeg",
+      "ffmpeg",
       [
         "-i",
         inputPath,