Add gallery admin and video media support.

This updates gallery handling to support video playback with generated poster thumbnails, adds authenticated admin upload/delete flows, and improves dev/runtime behavior including reliable thumbnail generation and media-safe response handling.

Co-authored-by: Cursor <cursoragent@cursor.com>

Dockerfile.dev

@@ -1,6 +1,10 @@
 ARG GO_VERSION=1.25
 FROM golang:${GO_VERSION}-bookworm
 
+RUN apt-get update \
+	&& apt-get install -y --no-install-recommends ffmpeg \
+	&& rm -rf /var/lib/apt/lists/*
+
 RUN go install github.com/a-h/templ/cmd/templ@latest \
 	&& go install github.com/air-verse/air@latest