feat: implement team routes and membership management

[Harxhit]

Summary

Closes #44

Implements backend support for Team / Group Cards as described in Product Doc 5.4.

This PR introduces team management APIs, public team profiles, membership management, and QR generation for team pages. Team creation now uses Prisma transactions to ensure atomic writes and prevent partial state.

Additionally, reusable slug utilities were introduced and integrated into the team flow, with related slug generation improvements also applied to events for consistency.

Schema

Added:

• Team
• TeamMember

Including:

• Team ↔ Owner relation
• Team ↔ TeamMember relation
• User ↔ Team membership relation
• Composite membership uniqueness constraint
• Indexing for Team.slug and TeamMember.userId

APIs

Implemented:

• POST /api/teams

  • authenticated team creation
  • automatic unique slug generation
  • owner membership creation

• GET /api/teams/:slug

  • public team profile
  • member profiles and platform links

• POST /api/teams/:slug/members

  • owner-only member management
  • duplicate and ownership validation

• DELETE /api/teams/:slug/members/:userId

  • owner removal
  • self-removal support

• PATCH /api/teams/:slug

  • owner-only team updates

• DELETE /api/teams/:slug

  • owner-only deletion

• GET /api/teams/:slug/qr

  • QR generation for team pages

Validation & Error Handling

Added:

• Zod validation for create/invite/update flows
• ownership guards
• authentication and authorization checks
• Prisma and QR generation error handling

TODO / Follow-up

• Owner self-removal is currently blocked.
• Ownership transfer / assigning next owner should be implemented before allowing owner leave/delete flows.