feat(webapp): stateful (CNPG-backed) WebApp shape in the KRO WebApp RGD

[devantler]

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Part of #1932 / #1933. Follow-up child from the Tenant-abstraction ADR (docs/tenant-abstraction.md).

Problem

The KRO WebApp archetype covers stateless Deployment-backed apps only. Database-backed apps (e.g. wedding-db, a CNPG cluster) fall outside it, so those tenants can't use the WebApp abstraction for their full shape.

Proposed direction

Extend the WebApp RGD with an optional stateful/database shape (a CNPG Cluster), so the archetype covers database-backed workloads. Reuse the platform's existing CNPG conventions (storage class, backup, WFFC scheduling) verbatim; render-diff vs a current CNPG-backed app to prove parity. Likely decompose further once scoped — this is the epic-level child.

Rough size

M–L — new schema shape + RGD template + parity render-diff; decompose into a first small increment when picked up.

Acceptance criteria

• WebApp schema supports an optional database shape that renders a CNPG Cluster matching current conventions.
• A CNPG-backed app render-diffs clean vs its pre-change manifests.
• Documented in TENANTS.md.