feat(api): adds Resource persistence model and migration

[eLRuLL]

Description

Introduces the initial persistence layer for the resource control-plane architecture.

Changes included:

• Add Resource model in core/models.py.
• Add migration core/migrations/0039_resource.py to create the Resource table.

Resource includes:

• Required scope: project
• Core state: kind, phase
• Control-plane payloads: desired_spec, observed_state, external_ref
• Reconciliation metadata: error_message, retry_count
• Links to existing entities from a reverse foreign key.
• Timestamps: created, updated

This PR is intentionally persistence-only:

• No dispatch/reconciler behavior changes
• No API contract changes
• No backfill/migration strategy logic yet

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. (not required for this persistence-only step)
• New and existing tests pass locally with my changes. (could not run here due missing local Django runtime in this environment)
• If this change is a core feature, I have added thorough tests. (not applicable in this persistence-only PR)
• 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.

[joaquingx]

There will be a conflict with this migration in #275, we need to coordinate it.

[joaquingx]

Will be good to add a str

[joaquingx]

e.g. spiderjob which kind of desired_spec should have, the same for observed_state ?