security: a disclosure policy, and Dependabot for the four dependency surfaces

.github/dependabot.yml

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+version: 2
+
+updates:
+  # Go modules (go.mod / go.sum at the repository root).
+  - package-ecosystem: gomod
+    directory: /
+    schedule:
+      interval: weekly
+
+  # The SvelteKit admin app. pnpm is handled by the npm ecosystem.
+  - package-ecosystem: npm
+    directory: /frontend
+    schedule:
+      interval: weekly
+
+  # Actions pinned in .github/workflows/.
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly
+
+  # Base images in the root Dockerfile.
+  - package-ecosystem: docker
+    directory: /
+    schedule:
+      interval: weekly

SECURITY.md

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+# Security policy
+
+## Supported versions
+
+Calnode is pre-1.0, and fixes go onto the latest release line only. There are no
+backports to earlier `0.x` minors.
+
+| Version | Supported |
+| ------- | --------- |
+| 0.9.x   | Yes       |
+| < 0.9   | No        |
+
+If you are running an older tag, the fix for a reported issue will be an upgrade.
+
+## Reporting a vulnerability
+
+Please report privately, through GitHub's private vulnerability reporting: open the
+repository's **Security** tab and use **Report a vulnerability**. That opens a private
+thread visible only to you and the maintainers.
+
+Please do not open a public issue, a pull request, or a discussion for a security
+report. A public issue is a disclosure, and it is one made before there is anything for
+people to upgrade to.
+
+## What to include
+
+Enough to reproduce it. Usually that is:
+
+- what the problem is, and what an attacker gets out of it;
+- the version, tag, or commit you tested;
+- how you are running it (Docker image, `go build`, behind which proxy) and anything
+  non-default in your configuration;
+- the steps, request, or proof-of-concept that triggers it;
+- what you expected to happen instead.
+
+If you are not sure whether something is a vulnerability, report it anyway and say so.
+An unclear report is easier to deal with than one that never arrives.
+
+## What to expect
+
+- **An acknowledgement within a few days.** If you have not heard anything after a week,
+  please post a follow-up on the same private thread in case it was missed.
+- **Then either a fix or an explanation.** If we agree it is a vulnerability, we will
+  tell you roughly when a fix will land and let you know when it ships. If we do not
+  think it is one, we will say why rather than leaving the report open.
+- **A public advisory when the fix is released**, through GitHub Security Advisories, so
+  people running Calnode know what they are upgrading for.
+- **Credit, if you want it.** Tell us the name or handle to use, or tell us you would
+  rather stay anonymous. Either is fine.
+
+Please give us a reasonable chance to ship a fix before publishing the details.