Add mergedrop: drag-and-drop video merger (ffmpeg + FastAPI + React)

apps/mergedrop/build/README.md

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+# MergeDrop image build
+
+The Dockerfile and full source for the **MergeDrop** image live in a separate Gitea repo:
+
+- Source: <https://git.alexzaw.dev/alexz/video-merger>
+- Published image: `git.alexzaw.dev/alexz/mergedrop:<tag>`
+
+This folder holds only a thin helper script (`build.sh`) that clones the source
+repo and pushes the resulting image to the in-house Docker registry.
+
+## Rebuild
+
+```bash
+cd build/
+./build.sh                                 # builds + pushes :latest and the version tag
+IMAGE_TAG=1.0.1 ./build.sh                 # override the tag
+```
+
+Requires `docker login git.alexzaw.dev` to already be done (or `~/.docker/config.json`
+to have the registry credentials, which is the case on this host).
+
+## Notes
+
+- The image is a two-stage build (Node 20 → Python 3.11-slim) and produces an
+  `amd64` image only. Add `--platform linux/arm64` to `build.sh` args to cross-build.
+- The application is a single FastAPI process that serves the static React build
+  and a `/api/*` JSON API on the same port (`8000` inside the container).
+- `ffmpeg` is installed in the runtime image; no host-level ffmpeg required.