feat: v3 Go rewrite — full artifact proxy with web UI, TUI, and Terraform provider

Complete rewrite of ArtifactAPI from Python/FastAPI to Go as a single binary.

Core engine:
- 10 package providers: generic, docker, helm, pypi, npm, rpm, alpine,
  puppet, terraform, goproxy — each with built-in mutable patterns
- Content-addressable storage (SHA256 dedup across all remotes)
- Three-tier caching: Redis (TTL/locks) → S3/MinIO (blobs) → upstream
- Classifier with allowlist/blocklist per-remote (empty = allow all)
- Circuit breaker, conditional revalidation, stale-on-error
- Background garbage collection for orphaned blobs
- Access logging to PostgreSQL

API:
- v1 proxy endpoints (backwards compatible)
- v2 management API: CRUD remotes/virtuals, object browser, stats,
  health, SSE events, probe/test endpoint
- Virtual repos with index merging (Helm YAML + PyPI HTML)

Frontend (React + Vite, separate Dockerfile):
- Dashboard with stats, health indicators, top remotes
- Remotes list with type filter, remote detail with config/patterns
- Object browser with pagination and evict
- Test Remote page: probe any remote path, see headers/size/timing
- Virtuals page with expandable member lists

TUI (Bubble Tea):
- Dashboard, remotes list/detail, object browser, virtuals
- Vim-style navigation, artifactapi tui --endpoint <url>

Infrastructure:
- S3 client supports MinIO, Ceph RGW, AWS S3 (minio-go)
- PostgreSQL schema with migrations
- Docker Compose: API + UI + Postgres 17 + Redis 7 + MinIO
- Makefile with Go version check, build/test/lint/fmt/e2e targets
- Distroless Docker image (~15MB)

Testing:
- Unit tests for models, classifier, providers, mergers
- E2E tests with testcontainers-go (real Postgres/Redis/MinIO)

Terraform config:
- All 40 production remotes + helm virtual as HCL
- Provider repo: terraform-provider-artifactapi v0.0.1 (separate)

internal/provider/goproxy/goproxy_test.go

@@ -0,0 +1,50 @@
+package goproxy_test
+
+import (
+	"testing"
+
+	"git.unkin.net/unkin/artifactapi/internal/provider"
+	"git.unkin.net/unkin/artifactapi/internal/provider/goproxy"
+	"git.unkin.net/unkin/artifactapi/pkg/models"
+)
+
+func TestProvider_Type(t *testing.T) {
+	p := &goproxy.Provider{}
+	if p.Type() != models.PackageGoProxy {
+		t.Errorf("expected goproxy, got %q", p.Type())
+	}
+}
+
+func TestProvider_Classify(t *testing.T) {
+	p := &goproxy.Provider{}
+	tests := []struct {
+		path string
+		want provider.Mutability
+	}{
+		{"golang.org/x/net/@v/list", provider.Mutable},
+		{"golang.org/x/net/@latest", provider.Mutable},
+		{"golang.org/x/net/@v/v0.1.0.info", provider.Immutable},
+		{"golang.org/x/net/@v/v0.1.0.mod", provider.Immutable},
+		{"golang.org/x/net/@v/v0.1.0.zip", provider.Immutable},
+	}
+	for _, tt := range tests {
+		if got := p.Classify(tt.path); got != tt.want {
+			t.Errorf("Classify(%q) = %v, want %v", tt.path, got, tt.want)
+		}
+	}
+}
+
+func TestProvider_ContentType(t *testing.T) {
+	p := &goproxy.Provider{}
+	tests := []struct{ path, want string }{
+		{"m/@v/v1.0.0.zip", "application/zip"},
+		{"m/@v/v1.0.0.mod", "text/plain"},
+		{"m/@v/v1.0.0.info", "application/json"},
+		{"m/@v/list", "text/plain"},
+	}
+	for _, tt := range tests {
+		if got := p.ContentType(tt.path); got != tt.want {
+			t.Errorf("ContentType(%q) = %q, want %q", tt.path, got, tt.want)
+		}
+	}
+}