fix: security hardening — block self-registration, SSE auth, audit logging fixes

Security fixes:
- Block self-registration via magic link (PrismaAdapter createUser throws)
- Magic links only sent to existing ACTIVE users (prevents enumeration)
- signIn callback rejects non-existent users (defense-in-depth)
- Change schema default role from JURY_MEMBER to APPLICANT
- Add authentication to live-voting SSE stream endpoint
- Fix false FILE_OPENED/FILE_DOWNLOADED audit events on page load
  (remove purpose from eagerly pre-fetched URL queries)

Bug fixes:
- Fix impersonation skeleton screen on applicant dashboard
- Fix onboarding redirect loop in auth layout

Observer dashboard redesign (Steps 1-6):
- Clickable round pipeline with selected round highlighting
- Round-type-specific dashboard panels (intake, filtering, evaluation,
  submission, mentoring, live final, deliberation)
- Enhanced activity feed with server-side humanization
- Previous round comparison section
- New backend queries for round-specific analytics

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

prisma/schema.prisma

@@ -302,7 +302,7 @@ model User {
   email          String     @unique
   name           String?
   emailVerified  DateTime? // Required by NextAuth Prisma adapter
-  role           UserRole   @default(JURY_MEMBER)
+  role           UserRole   @default(APPLICANT)
   roles          UserRole[] @default([])
   status         UserStatus @default(INVITED)
   expertiseTags  String[]   @default([])