urlaubsverwaltung: correct the env-collision claim, answer the recovery question

The chart told operators that adding a duplicate env name to an installed
release makes the next upgrade fail in the API server, quoting "The order in
patch list ... doesn't match $setElementOrder list". Measured against helm
3.19.0 and Kubernetes 1.35, it does not fail: the upgrade succeeds, helm prints
"Release has been upgraded", and Kubernetes merges the env list by name, so the
duplicate pair collapses to the chart's own entry and the operator's override is
discarded without a word. At install time the same values behave differently -
both entries reach the pod, the later one wins, and the API server says so with
"hides previous definition of ..., which may be dropped when using apply".

The advice that followed from the old claim survives - such a switch only takes
at install time - but the stated reason was wrong in the dangerous direction. A
reader who expects a loud failure treats a green upgrade as proof the override
took effect, and green is exactly what a silently dropped override looks like.
The text now names the real failure mode and tells the reader to read the value
back off the Deployment. README carries the same correction, because every chart
in this repo offers the same additive env block.

Also records what the earlier measurement left open. With the database scaled
back up the pod returns to Ready on its own after about five seconds, restart
count still 0, so leaving the liveness group untouched costs no manual recovery.
And the limit of that signal belongs next to it: the db indicator validates a
connection, not the schema, so a database that came back empty still put the pod
back to Ready with /actuator/health at 200.

README.md

@@ -245,6 +245,7 @@ These guidelines are **binding for every current and future chart** in this repo
 
 - Every workload container offers an additive `env` value block, appended to the env entries the chart always sets.
 - Entries are rendered through `tpl` (so values may contain Helm templating) and support the `value`, `secretKeyRef` and `configMapKeyRef` forms (pattern: `template-chart/templates/xxx.deployment.yaml`).
+- **Additive means additive: an entry repeating a name the chart already sets does not replace it, and what the pair does depends on when it arrives.** At install both entries reach the pod and the later one — the user's — wins. On an *existing* release the same entry is lost: Kubernetes merges `env` by name, the pair collapses to the chart's own value, and `helm upgrade` still reports success. Measured on `urlaubsverwaltung` (helm 3.19.0, Kubernetes 1.35): `helm get manifest` shows the variable twice while the live Deployment shows it once, carrying the chart's value. The failure mode is silence, so a chart whose `NOTES.txt` tells operators to override one of its own variables has to say that the override only takes at install time — and that a green upgrade proves nothing, the value belongs read back off the Deployment.
 
 ### 11. Scaling