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

migrations/012_remove_tags.sql

@@ -0,0 +1,33 @@
+-- Migration: Remove tag system
+-- Date: 2026-02-03
+-- Description: Remove tags table and related objects, keeping only versions for artifact references
+
+-- Drop triggers on tags table
+DROP TRIGGER IF EXISTS tags_ref_count_insert_trigger ON tags;
+DROP TRIGGER IF EXISTS tags_ref_count_delete_trigger ON tags;
+DROP TRIGGER IF EXISTS tags_ref_count_update_trigger ON tags;
+DROP TRIGGER IF EXISTS tags_updated_at_trigger ON tags;
+DROP TRIGGER IF EXISTS tag_changes_trigger ON tags;
+
+-- Drop the tag change tracking function
+DROP FUNCTION IF EXISTS track_tag_changes();
+
+-- Remove tag_constraint from artifact_dependencies
+-- First drop the constraint that requires either version or tag
+ALTER TABLE artifact_dependencies DROP CONSTRAINT IF EXISTS check_constraint_type;
+
+-- Remove the tag_constraint column
+ALTER TABLE artifact_dependencies DROP COLUMN IF EXISTS tag_constraint;
+
+-- Make version_constraint NOT NULL (now the only option)
+UPDATE artifact_dependencies SET version_constraint = '*' WHERE version_constraint IS NULL;
+ALTER TABLE artifact_dependencies ALTER COLUMN version_constraint SET NOT NULL;
+
+-- Drop tag_history table first (depends on tags)
+DROP TABLE IF EXISTS tag_history;
+
+-- Drop tags table
+DROP TABLE IF EXISTS tags;
+
+-- Rename uploads.tag_name to uploads.version (historical data field)
+ALTER TABLE uploads RENAME COLUMN tag_name TO version;