feat: add Docker support with self-hosted and hosted presets

Multi-stage Dockerfile (Node 22 Alpine), docker-compose for self-hosted
and hosted deployments, CI workflow for GHCR publishing, runtime env var
substitution, and cron sidecar with supercronic.

Aligns hosted extension preset with dev config (enable-banking,
ai-categorization, ai-chat, email).

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

docker/cron.Dockerfile

@@ -0,0 +1,13 @@
+FROM alpine:3.19
+
+ARG SUPERCRONIC_VERSION=v0.2.33
+ARG TARGETARCH
+
+RUN apk add --no-cache curl \
+    && ARCH=$(case ${TARGETARCH} in amd64) echo "linux-amd64";; arm64) echo "linux-arm64";; *) echo "linux-amd64";; esac) \
+    && curl -fsSL "https://github.com/aptible/supercronic/releases/download/${SUPERCRONIC_VERSION}/supercronic-${ARCH}" \
+       -o /usr/local/bin/supercronic \
+    && chmod +x /usr/local/bin/supercronic
+
+ENTRYPOINT ["supercronic"]
+CMD ["/etc/supercronic/crontab"]