Add Docker/systemd deployment, documentation, extauth logout URL wiring

Dockerfile

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+FROM golang:1.25-bookworm AS build
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+ARG VERSION=dev
+RUN CGO_ENABLED=0 go build -trimpath -ldflags "-s -w -X main.version=${VERSION}" -o /out/vibesonic ./cmd/vibesonic
+
+FROM debian:bookworm-slim
+RUN apt-get update && apt-get install -y --no-install-recommends ffmpeg ca-certificates \
+    && rm -rf /var/lib/apt/lists/* \
+    && useradd -r -u 10001 -d /data -s /usr/sbin/nologin vibesonic \
+    && mkdir -p /data /music \
+    && chown -R vibesonic:vibesonic /data
+
+COPY --from=build /out/vibesonic /usr/local/bin/vibesonic
+
+USER vibesonic
+VOLUME ["/data", "/music"]
+EXPOSE 8080
+
+ENV VIBESONIC_LISTEN_ADDR=:8080 \
+    VIBESONIC_DATA_DIR=/data \
+    VIBESONIC_DB=/data/vibesonic.db \
+    VIBESONIC_MUSIC_DIRS=/music \
+    VIBESONIC_FFMPEG_PATH=/usr/bin/ffmpeg \
+    VIBESONIC_FFPROBE_PATH=/usr/bin/ffprobe \
+    VIBESONIC_SCAN_INTERVAL=1h
+
+ENTRYPOINT ["/usr/local/bin/vibesonic"]