feat(api): add reconcile_rsource Celery task skeleton

[eLRuLL]

Description

Adds a skeleton Celery reconciler for the resource control-plane.

• New task core.tasks.reconcile_resource(resource_id) (name="core.tasks.reconcile_resource").
• Gated by RESOURCE_RECONCILER_ENABLED (default False). When disabled, returns immediately without touching the database.
• When enabled: loads Resource, exits early if missing or terminal phase; otherwise no-op (no Kubernetes, no state writes yet).
• Settings + .env.example document the flag.
• tests/core/test_reconcile_resource.py: disabled path, terminal phase, idempotent double-run on non-terminal, missing id.

Issue

• _N/A

Checklist before requesting a review

• I have performed a self-review of my code.
• My code follows the style guidelines of this project.
• I have made corresponding changes to the documentation. (.env.example only)
• New and existing tests pass locally with my changes. (run python manage.py test tests.core.test_reconcile_resource)
• If this change is a core feature, I have added thorough tests.
• If this change affects or depends on the behavior of other estela repositories, I have created pull requests with the relevant changes in the affected repositories. (not applicable)
• I understand that my pull request may be closed if it becomes obvious or I did not perform all of the steps above.

[github-actions]

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[joaquingx]

Schedule of the task isn't on the PR.