feat: [US-045] - Supabase Realtime channel and connection management

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

supabase/migrations/20260123200000_add_collaboration_sessions_unique_constraint.sql

@@ -0,0 +1,6 @@
+-- Add unique constraint on collaboration_sessions for (project_id, user_id)
+-- This ensures a user can only have one active session per project,
+-- and allows upsert operations for session management.
+ALTER TABLE collaboration_sessions
+  ADD CONSTRAINT collaboration_sessions_project_user_unique
+  UNIQUE (project_id, user_id);