Job backup strategy: make batch/v1.Job backoffLimit configurable #2742

Summary

The generic Job backup strategy (strategy.backups.cozystack.io/Job) renders the rendered batch/v1.Job with a hard-coded backoffLimit: 2 in buildJobStrategyBatchJob (internal/backupcontroller/jobstrategy_controller.go). Operators have no way to override it.

Why it matters

The Job strategy is the app-agnostic driver intended to wrap arbitrary backup tools. A fixed retry count is a poor fit across tools:

• An S3-heavy dump (e.g. clickhouse-backup) may want 0 retries — each retry re-uploads and is expensive.
• A sidecar-style tool may want more retries to ride out transient infrastructure errors.

The value of 2 was inherited from the Altinity driver, which can hard-code it because it wraps a single known tool. The Job driver cannot make that assumption.

Options

1. Preferred: expose backoffLimit as an optional field on JobSpec (api/backups/strategy/v1alpha1/job_types.go), defaulting to the current value of 2 when unset, and thread it into buildJobStrategyBatchJob. Requires a CRD regeneration.
2. Minimum: document in the JobSpec.Template CRD description that the rendered Job is fixed at backoffLimit=2 and cannot be overridden, so operators know to keep their backup image idempotent.

Context

Raised in review of #1721 (initial Job strategy implementation). That PR ships with the value fixed at 2; this issue tracks making it configurable.