feat: add admin interface for kudos and achievements

[whocoder11]

Implements #2 — full admin control panel with:

Backend (admin.js):

• GET /api/admin/kudos — paginated + search kudos
• DELETE /api/admin/kudos/:id — delete individual kudo
• GET /api/admin/badges — now includes holder counts
• POST /api/admin/badges/:slug/drop — remove badge from all users
• GET/POST/DELETE /api/admin/categories — full CRUD

Backend (app.js):

• Fix session cookie secure/sameSite to work on both HTTP and HTTPS

Frontend (AdminView.vue):

• Dashboard tab with overview stats and maintenance actions
• Users tab with role management and CRUD
• Bots tab with secret reveal/rotate/generate
• Kudos tab with search, pagination, and delete
• Badges tab with holder counts, drop from users, delete
• Grant Badge tab with user autocomplete
• Categories tab with full CRUD

[netlify]

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[lkocman]

I'm bit more in favor of CRD. But yes sometimes we have to fix typo in badge etc. Let me look into your work tomorrow. We had already an admin interface for everything, but kudos.