refactor: modular local provider interfaces (#52)

## Summary
Move package-type-specific local repo logic out of centralized handlers into provider packages via optional Go interfaces.

**New interfaces in `provider` package:**
- \`LocalUploader\`: \`ValidateUpload(filePath) → (storagePath, contentType, error)\` + \`UploadResponse(...)\`
- \`LocalIndexer\`: \`ServeLocalIndex(w, r, files, repoName, path) → bool\` + \`GenerateLocalIndex(ctx, files, repoName, path) → ([]byte, error)\`
- \`FileStore\`: \`ListFilesByPrefix\` + \`ListPackages\` (implemented by database.DB)

**Providers implement these interfaces:**
- PyPI: upload validation (wheel/sdist naming), simple index serving + generation
- Terraform: upload validation (provider zip naming), mirror protocol serving

**Handlers simplified to generic dispatch:**
- \`local.go\`: type-asserts to \`LocalUploader\`, falls back to generic upload
- \`proxy.go\`: type-asserts to \`LocalIndexer\`, falls back to raw file serving
- \`engine.go\`: type-asserts to \`LocalIndexer\` for local virtual members

Adding a new local repo type (e.g. RPM) = implement the interfaces in its provider package. Zero handler changes.

## Test plan
- [x] Build + unit tests pass
- [x] E2E: PyPI local upload → simple index → uv pip install (smoke test after refactor)

Reviewed-on: #52
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

internal/api/v1/proxy.go

@@ -6,8 +6,6 @@ import (
 	"io"
 	"log/slog"
 	"net/http"
-	"regexp"
-	"strings"
 
 	"github.com/go-chi/chi/v5"
 
@@ -17,11 +15,8 @@ import (
 	"git.unkin.net/unkin/artifactapi/internal/proxy"
 	"git.unkin.net/unkin/artifactapi/internal/storage"
 	"git.unkin.net/unkin/artifactapi/internal/virtual"
-	"git.unkin.net/unkin/artifactapi/pkg/models"
 )
 
-var semverRe = regexp.MustCompile(`^[0-9]+\.[0-9]+\.[0-9]+(?:-[a-zA-Z0-9.]+)?$`)
-
 type ProxyHandler struct {
 	engine        *proxy.Engine
 	virtualEngine *virtual.Engine
@@ -115,13 +110,9 @@ func (h *ProxyHandler) handleLocal(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	switch remote.PackageType {
-	case models.PackageTerraform:
-		if h.serveTerraformMirror(w, r, remote, path) {
-			return
-		}
-	case models.PackagePyPI:
-		if h.servePyPIMirror(w, r, remote, path) {
+	prov, _ := provider.Get(remote.PackageType)
+	if indexer, ok := prov.(provider.LocalIndexer); ok {
+		if indexer.ServeLocalIndex(w, r, h.db, remote.Name, path) {
 			return
 		}
 	}
@@ -129,49 +120,6 @@ func (h *ProxyHandler) handleLocal(w http.ResponseWriter, r *http.Request) {
 	h.serveLocalFile(w, r, localName, path)
 }
 
-func (h *ProxyHandler) serveTerraformMirror(w http.ResponseWriter, r *http.Request, remote *models.Remote, path string) bool {
-	parts := strings.Split(path, "/")
-	if len(parts) < 3 {
-		return false
-	}
-
-	namespace, typeName := parts[0], parts[1]
-	tail := parts[2]
-
-	if tail == "index.json" {
-		h.local.ServeTerraformIndex(w, r, remote.Name, namespace, typeName)
-		return true
-	}
-
-	if strings.HasSuffix(tail, ".json") {
-		version := strings.TrimSuffix(tail, ".json")
-		if semverRe.MatchString(version) {
-			h.local.ServeTerraformVersionDoc(w, r, remote.Name, namespace, typeName, version)
-			return true
-		}
-	}
-
-	return false
-}
-
-func (h *ProxyHandler) servePyPIMirror(w http.ResponseWriter, r *http.Request, remote *models.Remote, path string) bool {
-	if path == "simple" || path == "simple/" {
-		h.local.ServePyPIIndex(w, r, remote.Name)
-		return true
-	}
-
-	if strings.HasPrefix(path, "simple/") {
-		pkg := strings.TrimPrefix(path, "simple/")
-		pkg = strings.TrimSuffix(pkg, "/")
-		if pkg != "" && !strings.Contains(pkg, "/") {
-			h.local.ServePyPIPackageIndex(w, r, remote.Name, pkg)
-			return true
-		}
-	}
-
-	return false
-}
-
 func (h *ProxyHandler) serveLocalFile(w http.ResponseWriter, r *http.Request, repoName, path string) {
 	file, err := h.db.GetLocalFile(r.Context(), repoName, path)
 	if err != nil {