feat: add local RPM repository with on-demand repodata (#53)

## Summary
- Upload RPMs to local repos, metadata parsed async via cavaliergopher/rpm
- Repodata (repomd.xml, primary/filelists/other.xml.gz) generated on-demand from DB — nothing stored in S3
- RPM provider implements LocalUploader, PostUploadHook, and LocalIndexer
- New rpm_metadata table for parsed RPM header data (name, version, deps, etc.)
- New provider interfaces: PostUploadHook, BlobReader, MetadataStore, RPMMetadataReader

## Test plan
- [x] Upload cowsay RPM from epel → async metadata parse confirmed in logs
- [x] repomd.xml generated with correct hashes → primary.xml.gz has correct metadata
- [x] `dnf install` from local repo: download + install successful
- [x] Bad file rejection (.txt → 400), overwrite rejection (409)

Reviewed-on: #53
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

internal/api/v2/local.go

@@ -1,6 +1,7 @@
 package v2
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"io"
@@ -58,14 +59,14 @@ func (h *LocalHandler) upload(w http.ResponseWriter, r *http.Request) {
 	prov, _ := provider.Get(remote.PackageType)
 
 	if uploader, ok := prov.(provider.LocalUploader); ok {
-		h.uploadValidated(w, r, remote, filePath, uploader)
+		h.uploadValidated(w, r, remote, filePath, prov, uploader)
 		return
 	}
 
 	h.uploadGeneric(w, r, remote, filePath)
 }
 
-func (h *LocalHandler) uploadValidated(w http.ResponseWriter, r *http.Request, remote *models.Remote, filePath string, uploader provider.LocalUploader) {
+func (h *LocalHandler) uploadValidated(w http.ResponseWriter, r *http.Request, remote *models.Remote, filePath string, prov provider.Provider, uploader provider.LocalUploader) {
 	storagePath, contentType, err := uploader.ValidateUpload(filePath)
 	if err != nil {
 		http.Error(w, err.Error(), http.StatusBadRequest)
@@ -102,6 +103,10 @@ func (h *LocalHandler) uploadValidated(w http.ResponseWriter, r *http.Request, r
 		return
 	}
 
+	if hook, ok := prov.(provider.PostUploadHook); ok {
+		go hook.AfterUpload(context.Background(), remote.Name, storagePath, result.ContentHash, h, h.db)
+	}
+
 	writeJSON(w, http.StatusCreated, uploader.UploadResponse(storagePath, result.ContentHash, result.SizeBytes))
 }
 
@@ -190,3 +195,11 @@ func (h *LocalHandler) remove(w http.ResponseWriter, r *http.Request) {
 func (h *LocalHandler) DB() *database.DB {
 	return h.db
 }
+
+func (h *LocalHandler) Download(ctx context.Context, key string) (io.ReadCloser, int64, error) {
+	reader, info, err := h.store.Download(ctx, key)
+	if err != nil {
+		return nil, 0, err
+	}
+	return reader, info.Size, nil
+}