Scope backend Helm values under a backend block

[corneliusroemer]

Context: kubernetes/loculus/values.yaml is very flat, so backend-specific knobs sit beside global defaults. That makes it harder to see which values affect the backend and increases misconfiguration risk. Let’s start scoping backend-only settings under backend while leaving shared groupings (images/replicas/resources) unchanged for now.

Plan:

• Add backend.enabled (replacing disableBackend) and update templates that gate backend rendering/URL wiring (templates/loculus-backend.yaml, templates/loculus-backend-service.yaml, templates/loculus-preprocessing-deployment.yaml, templates/silo-deployment.yaml, templates/loculus-website-config.yaml) plus values.schema.json.
• Create backend.config (or similar) and move backend-only runtime values there, e.g. preprocessingTimeout, zstdCompressionLevel, and any other strictly backend toggles surfaced during the move. Keep accessionPrefix top-level (global).
• Keep existing shared blocks for images, replicas, and resources as-is in this phase.
• Update overlays that set these keys (values_e2e_and_dev.yaml, values_preview_server.yaml, etc.).
• Align values.schema.json with the new structure

Rationale: Scoping backend-only values under backend makes backend configuration more discoverable, reduces accidental cross-service overrides, and sets a pattern for later phases without disrupting current shared groupings.