Client-side optimization pipeline with apply-to-J-ID

Backend:
- POST /api/files/J-x/optimize/ applies a browser-processed file: replaces
  every copy (renaming when the extension changes), recomputes MD5, size
  and perceptual hashes, seeds guest visibility; 400 when identical,
  409 when the result matches another item, owner/staff only.

Frontend (no server-side processing by design):
- optimize.worker.ts + pipelines: Mediabunny/WebCodecs for video with a
  prefer-hardware hint and per-browser codec detection; MozJPEG/OxiPNG/
  libwebp (jSquash) for images; gifuct-js+gifenc and UPNG for GIF/APNG.
- OptimizeModal: per-file-type options, original vs processed previews
  with sizes/savings, progress bar with ETA, then Apply (overwrite).
- Optimize button on the library detail for the uploader/staff.

frontend/src/features/optimize/detect.ts

@@ -0,0 +1,32 @@
+import { canEncodeVideo, getEncodableVideoCodecs, type VideoCodec } from "mediabunny";
+
+export interface VideoCodecSupport {
+  codec: VideoCodec;
+  supported: boolean;
+  hardware: boolean;
+}
+
+const VIDEO_CODEC_CANDIDATES: VideoCodec[] = ["avc", "hevc", "vp9", "av1"];
+
+/** Which codecs this browser can encode, and whether hardware is preferred. */
+export async function detectVideoCodecs(): Promise<VideoCodecSupport[]> {
+  const supported = new Set(await getEncodableVideoCodecs());
+  const results: VideoCodecSupport[] = [];
+  for (const codec of VIDEO_CODEC_CANDIDATES) {
+    const encodable = supported.has(codec);
+    let hardware = false;
+    if (encodable) {
+      try {
+        hardware = await canEncodeVideo(codec, {
+          width: 1920,
+          height: 1080,
+          hardwareAcceleration: "prefer-hardware",
+        });
+      } catch {
+        hardware = false;
+      }
+    }
+    results.push({ codec, supported: encodable, hardware });
+  }
+  return results;
+}