feat(backend): bootstrap Go module, config, and data plane

Introduce configuration validation, Postgres migrations, Redis/storage adapters, and the seed sample video.

backend/Dockerfile

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+FROM golang:1.27-alpine AS builder
+
+WORKDIR /src
+
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+RUN CGO_ENABLED=0 GOOS=linux go build -o /out/pargar ./cmd/server
+
+FROM alpine:3.22
+
+RUN adduser -D -H -u 10001 app
+
+WORKDIR /app
+
+COPY --from=builder /out/pargar ./pargar
+COPY --from=builder /src/media/sample.mp4 ./media/sample.mp4
+RUN chown -R app:app /app
+
+USER app
+
+EXPOSE 8080
+
+CMD ["./pargar"]