feat: Setup CI/CD infrastructure with Go, Frontend, and Docker workflows (#22)

## Summary

This PR sets up the complete CI/CD infrastructure for the Chat Switchboard project, implementing automated testing, linting, building, and Docker containerization.

## Changes

### CI/CD Workflows

1. **`.gitea/workflows/backend.yml`** - Backend CI pipeline
   - Automated Go module initialization
   - Test execution with coverage reporting
   - Code linting with golangci-lint
   - Binary compilation with version tags
   - Artifact upload for debugging

2. **`.gitea/workflows/frontend.yml`** - Frontend CI pipeline
   - JavaScript linting with ESLint
   - CSS validation with Prettier
   - Standalone HTML build via build.sh
   - HTML structure validation
   - Build artifact management

3. **`.gitea/workflows/docker.yml`** - Docker CI pipeline
   - Backend container building and testing
   - Frontend container building
   - Automatic image tagging on tags/branches
   - Registry push on main branch
   - Multi-arch manifest creation

### Docker Configuration

- `server/Dockerfile` - Multi-stage Go backend container with health checks
- `Dockerfile.frontend` - Nginx frontend container with gzip compression
- `nginx.conf` - Optimized nginx config with security headers

### Dependencies

- `server/go.mod` - Initialized Go module with Gin and godotenv

### Documentation

- `docs/CICD_SETUP.md` - Comprehensive CI/CD documentation

## Features

- ✅ Auto-trigger on push/PR to main/develop
- ✅ Test coverage reporting
- ✅ Code quality checks (golangci-lint, ESLint)
- ✅ Build artifact management (7-day retention)
- ✅ Semantic versioning support (v* tags)
- ✅ Multi-stage Docker builds
- ✅ Container health checks
- ✅ Security headers in nginx
- ✅ Gzip compression
- ✅ Non-root container execution

## Testing

The workflows will automatically run on this PR. Once merged, all future PRs and pushes to main/develop will trigger the appropriate CI checks.

## Acceptance Criteria

- ✅ All PRs run CI checks
- ✅ Docker images auto-build on tags
- ✅ Standalone build generates on each commit

## Next Steps (Manual)

1. Merge this PR to `main`
2. Create `develop` branch: `git checkout main && git checkout -b develop && git push origin develop`
3. Configure branch protection in Gitea (Settings → Branches → Add protection rule for main/develop)
4. Test with a sample PR to verify CI runs

## Breaking Changes

None. This is purely infrastructure setup with no impact on existing functionality.

Reviewed-on: xcaliber/chat-switchboard#22

nginx.conf

@@ -0,0 +1,42 @@
+server {
+    listen 80;
+    server_name _;
+    root /usr/share/nginx/html;
+    index index.html;
+
+    # Backend API proxy
+    location /api/ {
+        proxy_pass http://localhost:8080;
+        proxy_http_version 1.1;
+        proxy_set_header Upgrade $http_upgrade;
+        proxy_set_header Connection 'upgrade';
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+        proxy_cache_bypass $http_upgrade;
+    }
+
+    # Gzip compression
+    gzip on;
+    gzip_vary on;
+    gzip_min_length 1024;
+    gzip_proxied expired no-cache no-store private auth;
+    gzip_types text/plain text/css text/xml text/javascript application/x-javascript application/xml application/javascript;
+
+    # Cache static assets
+    location ~* \.(jpg|jpeg|png|gif|ico|css|js|woff|woff2)$ {
+        expires 1y;
+        add_header Cache-Control "public, immutable";
+    }
+
+    # SPA fallback
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+
+    # Security headers
+    add_header X-Frame-Options "SAMEORIGIN" always;
+    add_header X-Content-Type-Options "nosniff" always;
+    add_header X-XSS-Protection "1; mode=block" always;
+}