[FEATURE] Public fork/remix provenance and branching backend

[aernw]

Objective

Create the backend model for public forking and remix lineage.

Scope

• provenance between source projects and derived remix/fork projects
• backend branching semantics for public remix entry points
• lineage data that can be shown in public project surfaces later

Out of Scope

• [Features explicitly not included]
• [Future enhancements to defer]

Acceptance Criteria

• remix relationships are explicit and traceable
• fork/remix UX can rely on a real backend provenance model

Dependencies

Blocked by: (none)

Blocks: (none)

Success Metrics

• [How to measure success - optional]

Roadmap Context

Milestone: M6 - Productization & Coverage Expansion
Parent workstream(s): #163, #211