Add organization model and memberships to core service

[geoffjay]

Context

The core service (#211) requires organization support with a many-to-many relationship to users through a memberships table. This builds on the user model (#213).

Acceptance Criteria

• Create Organization model struct with fields:
  • id (UUID, primary key)
  • name (TEXT, unique, not null)
  • created_at (TEXT/datetime)
  • updated_at (TEXT/datetime)
• Create Membership model struct with fields:
  • id (UUID, primary key)
  • user_id (UUID, foreign key to users)
  • organization_id (UUID, foreign key to organizations)
  • role (TEXT, e.g. "owner", "member")
  • created_at (TEXT/datetime)
• Create OrganizationStorage with CRUD operations
• Create MembershipStorage with operations:
  • add_member(org_id, user_id, role)
  • remove_member(org_id, user_id)
  • list_members(org_id) — returns users in an organization
  • list_user_organizations(user_id) — returns organizations a user belongs to
  • get_membership(org_id, user_id) — check membership/role
• Add unique constraint on (user_id, organization_id) in memberships
• Add foreign key constraints with ON DELETE CASCADE
• Add unit tests for all storage operations including edge cases
• Ensure when a user's active organization is deleted, it's set to NULL

Relevant Files

• crates/orchestrator/src/storage.rs — reference for storage patterns
• New: crates/core/src/models/organization.rs, crates/core/src/storage/organizations.rs, crates/core/src/storage/memberships.rs

Dependencies

• Blocked by Add user model and SQLite storage to core service #213 (user model)