feat(keep): add opt-in PodDisruptionBudget support

[somaz94]

Adds an opt-in PodDisruptionBudget for the three HA components (backend, frontend, websocket), each of which already ships an HPA. Disabled by default; when enabled it defaults to minAvailable: 1 with an optional maxUnavailable override.

Validation:

• helm template: 3 PDBs render when enabled, 0 by default, maxUnavailable override omits minAvailable correctly
• ct lint (yamale + yamllint + helm lint) all passed; chart version bumped 0.1.96 -> 0.1.97
• helm-docs v1.14.2 values-table rows added for the new keys

Closes #109

[somaz94]

@talboren gentle ping — #199 (opt-in PDB) and #198 (Gateway API HTTPRoute) have been open since July without a review. I saw #202 merge last week, so these may just be sitting in a blind spot. Happy to rebase #199 onto the current base if that helps.