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

apps/mergedrop/metadata/description.md

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+# MergeDrop
+
+A self-hosted, drag-and-drop **video merger**. Drop clips → reorder → click **Merge & Build MP4** → download a single ready-to-share file.
+
+## Why this exists
+
+Most "merge video" tools are bloated desktop apps, paid SaaS, or sketchy converter sites that re-upload your footage to who-knows-where. MergeDrop is the opposite:
+
+- 100% self-hosted, runs in a single container on your Runtipi
+- No accounts, no telemetry, no third-party upload caps
+- Mixed formats / resolutions / framerates → normalized to a clean **1080p30 h.264 + AAC** MP4
+
+## Features
+
+- Drag-and-drop upload, drag-to-reorder, drag-to-remove
+- Live ffmpeg progress (percentage + tail of stderr) per merge job
+- In-browser preview of the merged result, then one-click download
+- Built-in janitor — old outputs are auto-deleted after a configurable retention window
+
+## Supported inputs
+
+`mp4`, `mov`, `mkv`, `avi`, `webm`, `m4v`, `flv`, `wmv`, `mpg/mpeg` (mixed codecs and dimensions are normalized via two-pass ffmpeg pipeline).
+
+## Output presets
+
+`1080p` (default), `720p`, `4K`, `vertical 9:16`, `square 1:1` — selectable per job from the UI.
+
+## Configuration
+
+| Setting | Default | Description |
+| --- | --- | --- |
+| Max files per merge | `20` | Hard cap per job |
+| Max per-file size | `2 GiB` | Per-upload limit (bytes) |
+| Output retention | `24h` | Janitor sweeps merged MP4s older than this |
+
+## Volumes (persistent)
+
+- `${APP_DATA_DIR}/data/uploads` — staged source clips
+- `${APP_DATA_DIR}/data/outputs` — merged MP4 results
+- `${APP_DATA_DIR}/data/thumbs` — generated upload thumbnails
+
+## Tech stack
+
+- Backend: FastAPI + `ffmpeg` subprocess (async, streamed progress)
+- Frontend: React 19 + Tailwind + `@dnd-kit` + `react-dropzone`
+- Single container, single port, no database