[Feature]: Limit project visibility to public only for free usage

[MAY55A]

Problem statement

Users can currently select private/restricted visibilities, which conflicts with the current free-tier policy.

Proposed solution

• In project create/edit UI, allow only public visibility.
• Hide or disable private and restricted options.
• Enforce the same rule server-side (API/DB validation) so non-public values cannot be persisted.
• Add helper text indicating private/restricted options are planned for a future release.

Alternatives considered (optional)

• Keep options visible but gated with upgrade messaging.
• Allow selection but block on submit.
  Preferred approach: disable/hide options + backend enforcement to avoid inconsistent states.

Users / impact

• End users get clear and consistent behavior aligned with current free-tier capabilities.
• Reduces confusion and avoids unsupported configurations.

Scope and constraints (optional)

• In scope: UI visibility selector behavior + server-side validation.
• Out of scope: introducing billing/plan-based visibility controls in this issue.
• Constraint: preserve compatibility for existing public project workflows.

Acceptance criteria

• Clear user-facing outcome defined
• Technical approach agreed
• Success can be validated in QA/UAT
• Only public visibility can be selected in relevant forms
• API/DB layer rejects non-public visibility values in this phase
• Existing public project flows remain unaffected

Additional context (optional)

Related to initial launch constraints for free usage.