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

server/go.mod

@@ -0,0 +1,35 @@
+module git.gobha.me/xcaliber/chat-switchboard
+
+go 1.21
+
+require (
+	github.com/gin-gonic/gin v1.9.1
+	github.com/joho/godotenv v1.5.1
+)
+
+require (
+	github.com/bytedance/sonic v1.9.1 // indirect
+	github.com/chenzhuoyu/base64x v0.0.0-20221115062448-fe3a3abad311 // indirect
+	github.com/gabriel-vasile/mimetype v1.4.2 // indirect
+	github.com/gin-contrib/sse v0.1.0 // indirect
+	github.com/go-playground/locales v0.14.1 // indirect
+	github.com/go-playground/universal-translator v0.18.1 // indirect
+	github.com/go-playground/validator/v10 v10.14.0 // indirect
+	github.com/goccy/go-json v0.10.2 // indirect
+	github.com/json-iterator/go v1.1.12 // indirect
+	github.com/klauspost/cpuid/v2 v2.2.4 // indirect
+	github.com/leodido/go-urn v1.2.4 // indirect
+	github.com/mattn/go-isatty v0.0.19 // indirect
+	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
+	github.com/modern-go/reflect2 v1.0.2 // indirect
+	github.com/pelletier/go-toml/v2 v2.0.8 // indirect
+	github.com/twitchyliquid64/golang-asm v0.15.1 // indirect
+	github.com/ugorji/go/codec v1.2.11 // indirect
+	golang.org/x/arch v0.3.0 // indirect
+	golang.org/x/crypto v0.14.0 // indirect
+	golang.org/x/net v0.17.0 // indirect
+	golang.org/x/sys v0.13.0 // indirect
+	golang.org/x/text v0.13.0 // indirect
+	google.golang.org/protobuf v1.30.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)