feat(admin): module skeleton, config and two database connections

Adds the admin module: env config with fail-fast validation, two MongoDB
connections (its own vantage_admin database plus a narrow path into the
control plane), the boot sequence and the image.

Config refuses to start without a signing key, and both Mongo URIs must
name their database inline -- admin talks to two databases, so a bare
MONGO_DB would be ambiguous about which.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

admin/Dockerfile

@@ -0,0 +1,30 @@
+# Context is the repository root; admin depends on the shared module.
+FROM golang:1.26-alpine AS builder
+
+WORKDIR /src
+
+COPY shared/go.mod shared/go.sum ./shared/
+COPY admin/go.mod admin/go.sum ./admin/
+RUN cd admin && go mod download
+
+COPY shared/ ./shared/
+COPY admin/ ./admin/
+
+RUN cd admin && CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -o /out/admin ./cmd
+RUN cd admin && CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -o /out/adminctl ./cmd/adminctl
+
+FROM alpine:3.20 AS runner
+
+RUN apk add --no-cache ca-certificates && \
+    addgroup --system --gid 1001 admin && \
+    adduser --system --uid 1001 --ingroup admin admin
+
+COPY --from=builder /out/admin /usr/local/bin/admin
+COPY --from=builder /out/adminctl /usr/local/bin/adminctl
+
+USER admin
+
+EXPOSE 8083
+ENV PORT=8083
+
+CMD ["/usr/local/bin/admin"]