docs: add staging deployment troubleshooting guide

Comprehensive documentation of the staging deployment journey including:

- Problem 1: GitHub workflow file extensions (.yml.bak to disable)
- Problem 2: chat-backend health check path (/api/v1/health not /api/health)
- Problem 3: SvelteKit static env imports (use runtime patterns for Docker)
- Problem 4: Orphan Docker containers

Also fixes the cd-staging.yml health check path for chat-backend to match
the actual NestJS endpoint at /api/v1/health.

Includes checklists, debugging commands, and lessons learned.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.github/workflows/cd-staging.yml

@@ -216,7 +216,7 @@ jobs:
 
             # Check chat-backend
             echo "Checking chat-backend..."
-            if docker compose exec -T chat-backend wget -q -O - http://localhost:3002/api/health > /dev/null 2>&1; then
+            if docker compose exec -T chat-backend wget -q -O - http://localhost:3002/api/v1/health > /dev/null 2>&1; then
               echo "✅ chat-backend is healthy"
             else
               echo "❌ chat-backend health check failed"