renovate: keep the delegation nginx on stable branches

nginx numbers its release branches by minor: even is stable, odd is
mainline. PR #139 proposed 1.30.4 -> 1.31.4, which would have moved the
Synapse delegation sidecar onto mainline without that ever being a
decision. allowedVersions now permits even minors only; patch updates
inside the stable branch still come through and still automerge.

README.md

@@ -70,6 +70,8 @@ What Renovate keeps current:
 
 Each chart renders its main image as `<repo>:{{ .Chart.AppVersion }}` — repository in the template, version in `Chart.yaml`. Renovate cannot join two files, so the mapping lives in `renovate.json5`, one block per chart. It is deliberately *not* a `# renovate:` comment inside each `Chart.yaml`: that would touch all chart directories and force a version bump plus a publish per chart for a pure comment change. Charts whose tags carry a flavour or date suffix (`apache-tika`, `minecraft-server`, `rss-bridge`, `teamspeak-server`) override `versioning` in their block; the reasoning sits next to each one.
 
+One dependency is constrained beyond its versioning: **nginx** (the Synapse delegation sidecar) is limited to **even minors** via `allowedVersions`. nginx numbers its release branches by minor — even is stable, odd is mainline — and the chart belongs on stable. Patch updates inside the stable branch still flow through normally.
+
 Not managed: `ci/**` (install-test fixtures pin throwaway infrastructure to major lines on purpose) and `template-chart/**` (placeholder image). A consequence worth knowing: `ci/matrix-synapse/fixtures.yaml` pins a Synapse image that Renovate will not follow — bump it by hand when the chart moves a major.
 
 ### The version-bump hook