fix: remediate severity-ordered review findings across cache, scanners, and proxies

- H2: gormstore soft-delete + storage_key UNIQUE collision; re-save after eviction succeeds
- H1: cache getOrFetch cache-hit branch no longer runs CheckVulnerabilities on metadata entries
- M2: cache store() streams upstream body to temp file + SHA256 instead of double-buffering
- H3/L3: FetchResult/FetchFunc plumbing (+RequiresAuth/AuthProvider); proxy closures adapted;
        goproxy strips git+ prefix from UpstreamURL
- H4: scanner normalizes bypass package names; CheckVulnerabilities builds normalized packageKey
- M4: requireAdmin routes through extractAPIKey -> ValidateAPIKey -> admin role check
- M5: network client keeps only final RecordFailure (one count per attempt cycle)
- M3: normalizeOriginHost + buildCheckOrigin rewrite; origin_test.go added
- M1: app lifecycle cancel func; Run -> Shutdown on listen error (sole cache owner); JoinHostPort
- M6: VulnerabilityThresholds defaults all -1 so BlockOnSeverity governs
- L4: metadata.NormalizeSeverity at 4 severity-count sites (MEDIUM->MODERATE consistency)
- L1: handleDeleteBypass uses structured *errors.Error + ErrCodeNotFound
- L2: pypi extractVersionFromFilename (wheel/2nd segment, non-wheel/last digit segment);
      cacheVersion uses full unique basename; regression tests added
- L5: document per-instance ValidationCache key scoping; rename packageURL param to resource

All: go build ./... && go vet ./... && go test ./... pass; golangci-lint clean.

pkg/metadata/gormstore/gormstore_v2.go

@@ -265,8 +265,22 @@ func (s *GORMStoreV2) SavePackage(ctx context.Context, pkg *metadata.Package) er
 			return errors.Wrap(result.Error, errors.ErrCodeStorageFailure, "failed to update package")
 		}
 
-		// If no rows were updated, create new record
+		// If no rows were updated, create new record.
+		// RowsAffected == 0 here means no LIVE row matches the key, because
+		// GORM's default scope appends `deleted_at IS NULL` to the UPDATE.
+		// The only possible match is a soft-deleted row (e.g. after TTL/evict
+		// called DeletePackage). That retained row still holds the same
+		// `storage_key`, which has a UNIQUE index, so a plain Create would
+		// collide (UNIQUE constraint failed: packages.storage_key) and the
+		// re-fetch would permanently 502. Hard-delete any such stale row
+		// before creating.
 		if result.RowsAffected == 0 {
+			if err := tx.Unscoped().
+				Where("registry_id = ? AND name = ? AND version = ?", registryID, pkg.Name, pkg.Version).
+				Delete(&PackageModel{}).Error; err != nil {
+				return errors.Wrap(err, errors.ErrCodeStorageFailure, "failed to purge stale soft-deleted package before create")
+			}
+
 			if err := tx.Create(model).Error; err != nil {
 				return errors.Wrap(err, errors.ErrCodeStorageFailure, "failed to create package")
 			}

pkg/metadata/gormstore/gormstore_v2_test.go

@@ -209,6 +209,57 @@ func (s *GORMStoreV2TestSuite) Test_V2_DeletePackage_Success() {
 	s.Equal(int64(1), count) // Still in DB, just soft deleted
 }
 
+// Test_V2_SavePackage_AfterSoftDelete tests that saving the same
+// registry/name/version key after a soft delete (e.g. TTL expiry or
+// cache eviction) succeeds instead of colliding on the retained
+// storage_key UNIQUE index. Regression for the permanent 502 on
+// re-fetch of a once-evicted package.
+func (s *GORMStoreV2TestSuite) Test_V2_SavePackage_AfterSoftDelete() {
+	pkg := &metadata.Package{
+		Registry:     "npm",
+		Name:         "re-add-test",
+		Version:      "1.0.0",
+		StorageKey:   "npm/re-add-test/1.0.0.tgz",
+		Size:         1000,
+		CachedAt:     time.Now(),
+		LastAccessed: time.Now(),
+	}
+	err := s.store.SavePackage(s.ctx, pkg)
+	s.NoError(err)
+
+	// Soft-delete (simulates TTL expiry / eviction)
+	err = s.store.DeletePackage(s.ctx, "npm", "re-add-test", "1.0.0")
+	s.NoError(err)
+
+	// Re-save the same key (simulates re-fetch after cache miss).
+	// Before the fix this failed with:
+	//   UNIQUE constraint failed: packages.storage_key
+	pkg2 := &metadata.Package{
+		Registry:     "npm",
+		Name:         "re-add-test",
+		Version:      "1.0.0",
+		StorageKey:   "npm/re-add-test/1.0.0.tgz",
+		Size:         2000,
+		CachedAt:     time.Now(),
+		LastAccessed: time.Now(),
+	}
+	err = s.store.SavePackage(s.ctx, pkg2)
+	s.NoError(err)
+
+	// The re-saved package must be retrievable
+	retrieved, err := s.store.GetPackage(s.ctx, "npm", "re-add-test", "1.0.0")
+	s.NoError(err)
+	s.NotNil(retrieved)
+	s.Equal(int64(2000), retrieved.Size)
+
+	// No duplicate rows retained after the hard purge
+	var count int64
+	s.store.db.Unscoped().Model(&PackageModel{}).
+		Where("name = ?", "re-add-test").
+		Count(&count)
+	s.Equal(int64(1), count)
+}
+
 // Test_V2_ListPackages_All tests listing all packages
 func (s *GORMStoreV2TestSuite) Test_V2_ListPackages_All() {
 	// Create multiple packages