feat: allow configuring the Deployment update strategy

[briferz]

Summary

Adds a new backstage.strategy value that maps to the Deployment spec.strategy field so operators can opt into Recreate (or RollingUpdate with maxSurge: 0) and guarantee that only one Backstage pod runs at a time.

Default behavior is unchanged: when backstage.strategy is unset (the default {}), no strategy block is rendered and Kubernetes keeps applying its own default (RollingUpdate with maxSurge: 25% / maxUnavailable: 25%).

Motivation

Backstage plugins (e.g. catalog, auth) run knex database migrations during plugin initialization on every pod startup. Under the current default RollingUpdate strategy, a new pod starts while the old pod is still running, and both race the knex_migrations_lock row. If either pod is killed mid-migration (OOM, node drain, probe failure during the rollout), the lock stays set (is_locked = 1) and every subsequent pod fails catalog startup with:

MigrationLocked: Migration table is already locked
If you are sure migrations are not running you can release the lock manually
by running 'knex migrate:unlock'

Recovery requires manual DB intervention — not great on production.

Allowing operators to pin the strategy to Recreate (or RollingUpdate with maxSurge: 0) removes the concurrent-migration race entirely for single-replica deployments (which is the common case for Backstage).

Usage

backstage:
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxSurge: 0
      maxUnavailable: 1

or

backstage:
  strategy:
    type: Recreate

Changes

• values.yaml: add backstage.strategy (default {}) with helm-docs comment.
• values.schema.tmpl.json: add JSON schema entry with examples; values.schema.json regenerated by the jsonschema-dereference pre-commit hook.
• templates/backstage-deployment.yaml: conditionally render spec.strategy from .Values.backstage.strategy using common.tplvalues.render so templated values (e.g. global settings) are supported.
• ci/strategy-values.yaml: new CI test case (RollingUpdate with maxSurge: 0).
• Chart.yaml: bump chart version 2.6.3 → 2.7.0 (minor — new additive feature, backward-compatible).
• README.md: regenerated by helm-docs.

Test plan

• pre-commit run --all-files passes (helm-docs, jsonschema-dereference, codespell).
• ct lint --charts charts/backstage passes against all ci/*-values.yaml files including the new strategy-values.yaml.
• helm template with default values produces no strategy block in the rendered Deployment (preserves existing behavior).
• helm template with ci/strategy-values.yaml correctly renders the strategy block with maxSurge: 0, maxUnavailable: 1.
• Commit is GPG-signed and DCO signed-off per CONTRIBUTING.md.