Convert backend from submodule to regular directory

Remove submodule tracking; backend is now a plain directory in the repo.
Also update deploy workflow: remove --recurse-submodules.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/Dockerfile

@@ -0,0 +1,12 @@
+FROM golang:1.25-alpine AS builder
+WORKDIR /app
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN CGO_ENABLED=0 GOOS=linux go build -o /server ./cmd/server
+RUN CGO_ENABLED=0 GOOS=linux go build -o /createuser ./cmd/createuser
+
+FROM gcr.io/distroless/static-debian12
+COPY --from=builder /server /server
+COPY --from=builder /createuser /createuser
+ENTRYPOINT ["/server"]