chore(renovate): add SOPS round-trip postUpgradeTasks + self-hosted runner

Address review feedback on SOPS handling for encrypted values.yaml.

- renovate.json: Renovate has NO preUpgradeTasks option (verified against
  the renovate-schema). Decrypt must therefore happen inside postUpgradeTasks
  (which run after the tag bump, before the commit): decrypt the values.yaml,
  then re-encrypt. Encrypting an already-encrypted file would double-encrypt
  the existing ENC secret values. Only **/values.yaml is re-encrypted; the
  docker-compose and Chart.yaml files are not SOPS-encrypted here.
- kubernetes/code/renovate: self-hosted Renovate runner as a Flux CronJob
  running the renovatebot/renovate image, with sops allowed as a
  postUpgradeTask command and the SOPS PGP key loaded to re-encrypt
  values.yaml. Secrets come from SOPS-encrypted dotenv/env file sources.
- Wire code/renovate into the root kubernetes kustomization.

kubernetes/code/renovate/config.json

@@ -0,0 +1,11 @@
+{
+  "platform": "github",
+  "repositories": ["vhaudiquet/homeprod"],
+  "autodiscover": false,
+  "allowedPostUpgradeCommands": ["^sops"],
+  "allowedPostUpgradeCommandsEnv": ["^SOPS_"],
+  "onboardingConfigFileName": "renovate.json",
+  "dryRun": false,
+  "binarySource": "docker",
+  "schedule": ["every weekend"]
+}