docs(self-hosting): cover ClickHouse upgrade edge cases

Adds the recovery path for GitOps renders that missed existingClaim,
the pinned Bitnami image note for the Helm chart, the single-node
clustering note, and the one-way rollback caveat for Docker Compose.

Author: matt-aitken

docs/self-hosting/docker.mdx

@@ -368,7 +368,9 @@ You can also lock the versions of the bundled services, for example with `CLICKH
   The bundled ClickHouse now uses the official `clickhouse/clickhouse-server` image. Your existing
   data volume carries over automatically. If you previously pinned `CLICKHOUSE_IMAGE_TAG` to a
   Bitnami tag (for example `25.7.5-debian-12-r0`), update it to an official image tag such as
-  `26.2` — Bitnami tags don't exist in the official repository.
+  `26.2` — Bitnami tags don't exist in the official repository. Note the switch is one-way: the
+  official image takes ownership of the data files, so rolling back to the Bitnami image requires
+  manually restoring their previous owner (`chown -R 1001:1001` on the volume).
 </Note>
 
 <Note>

docs/self-hosting/kubernetes.mdx

@@ -287,7 +287,16 @@ redis:
   manifests without cluster access (for example with GitOps tools that use `helm template`), set
   `clickhouse.persistence.existingClaim` to the old PVC name
   (`data-<release>-clickhouse-shard0-0`) to keep your data — auto-detection can't run there, and
-  skipping this starts ClickHouse on a fresh empty volume.
+  skipping this starts ClickHouse on a fresh empty volume. If that happened, your old data is
+  still on the old PVC: delete the ClickHouse StatefulSet with `--cascade=orphan` (its volume
+  configuration is immutable), set `existingClaim`, and sync again.
+</Note>
+
+<Note>
+  If you pinned `clickhouse.image` to a Bitnami repository or tag in your values, update it to
+  the official `clickhouse/clickhouse-server` image — Bitnami tags don't exist there. The
+  bundled ClickHouse is single-node: the old Bitnami subchart keys (`shards`, `replicaCount`,
+  `keeper`) are no longer supported, so use an external ClickHouse for clustered setups.
 </Note>
 
 <Note>