Remove tag system, use versions only for artifact references

Tags were mutable aliases that caused confusion alongside the immutable
version system. This removes tags entirely, keeping only PackageVersion
for artifact references.

Changes:
- Remove tags and tag_history tables (migration 012)
- Remove Tag model, TagRepository, and 6 tag API endpoints
- Update cache system to create versions instead of tags
- Update frontend to display versions instead of tags
- Remove tag-related schemas and types
- Update artifact cleanup service for version-based ref_count

frontend/src/pages/ProjectPage.tsx

@@ -349,9 +349,9 @@ function ProjectPage() {
               render: (pkg: Package) => <Badge variant="default">{pkg.format}</Badge>,
             }] : []),
             ...(!project?.is_system ? [{
-              key: 'tag_count',
-              header: 'Tags',
-              render: (pkg: Package) => pkg.tag_count ?? '—',
+              key: 'version_count',
+              header: 'Versions',
+              render: (pkg: Package) => pkg.version_count ?? '—',
             }] : []),
             {
               key: 'artifact_count',
@@ -365,10 +365,10 @@ function ProjectPage() {
                 pkg.total_size !== undefined && pkg.total_size > 0 ? formatBytes(pkg.total_size) : '—',
             },
             ...(!project?.is_system ? [{
-              key: 'latest_tag',
+              key: 'latest_version',
               header: 'Latest',
               render: (pkg: Package) =>
-                pkg.latest_tag ? <strong style={{ color: 'var(--accent-primary)' }}>{pkg.latest_tag}</strong> : '—',
+                pkg.latest_version ? <strong style={{ color: 'var(--accent-primary)' }}>{pkg.latest_version}</strong> : '—',
             }] : []),
             {
               key: 'created_at',