Project concept: aggregate generations into projects/sets

[thedavidweng]

Problem

Every generation is a standalone item. No way to group related generations (e.g., variations of the same track, album drafts, project iterations).

Scope

• Introduce a "Project" concept: a named group that aggregates multiple generations
• DB schema: projects table + generations.project_id foreign key
• Sidebar "Projects" group above "History"
• Assign/unassign generations to projects
• CLI --project flag for openloop run and openloop list

Acceptance criteria

• Users can create, rename, and delete projects
• Generations can be assigned to a project from history
• Sidebar shows a Projects section above History
• openloop run --project <name> groups the output under that project
• openloop list --project <name> filters by project