[FEATURE] Project template model and starter-session publishing flow

[aernw]

Objective

Create the backend model for reusable templates and starter sessions.

Scope

• template metadata and publishing rules
• starter-session creation and reuse semantics
• data structures that support template browsing and future curation

Out of Scope

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

Acceptance Criteria

• templates are first-class reusable objects rather than renamed projects
• frontend template browsing can rely on a stable backend model

Dependencies

Blocked by: (none)

Blocks: (none)

Success Metrics

• [How to measure success - optional]

Roadmap Context

Milestone: M6 - Productization & Coverage Expansion
Parent workstream(s): #212