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

backend/app/pypi_proxy.py

@@ -20,7 +20,7 @@ from fastapi.responses import StreamingResponse, HTMLResponse
 from sqlalchemy.orm import Session
 
 from .database import get_db
-from .models import UpstreamSource, CachedUrl, Artifact, Project, Package, Tag, PackageVersion
+from .models import UpstreamSource, CachedUrl, Artifact, Project, Package, PackageVersion
 from .storage import S3Storage, get_storage
 from .config import get_env_upstream_sources
 
@@ -646,20 +646,6 @@ async def pypi_download_file(
             db.add(package)
             db.flush()
 
-        # Create tag with filename
-        existing_tag = db.query(Tag).filter(
-            Tag.package_id == package.id,
-            Tag.name == filename,
-        ).first()
-        if not existing_tag:
-            tag = Tag(
-                package_id=package.id,
-                name=filename,
-                artifact_id=sha256,
-                created_by="pypi-proxy",
-            )
-            db.add(tag)
-
         # Extract and create version
         # Only create version for actual package files, not .metadata files
         version = _extract_pypi_version(filename)