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/models.py

@@ -71,7 +71,6 @@ class Package(Base):
     )
 
     project = relationship("Project", back_populates="packages")
-    tags = relationship("Tag", back_populates="package", cascade="all, delete-orphan")
     uploads = relationship(
         "Upload", back_populates="package", cascade="all, delete-orphan"
     )
@@ -120,7 +119,6 @@ class Artifact(Base):
     ref_count = Column(Integer, default=1)
     s3_key = Column(String(1024), nullable=False)
 
-    tags = relationship("Tag", back_populates="artifact")
     uploads = relationship("Upload", back_populates="artifact")
     versions = relationship("PackageVersion", back_populates="artifact")
     dependencies = relationship(
@@ -151,65 +149,6 @@ class Artifact(Base):
     )
 
 
-class Tag(Base):
-    __tablename__ = "tags"
-
-    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
-    package_id = Column(
-        UUID(as_uuid=True),
-        ForeignKey("packages.id", ondelete="CASCADE"),
-        nullable=False,
-    )
-    name = Column(String(255), nullable=False)
-    artifact_id = Column(String(64), ForeignKey("artifacts.id"), nullable=False)
-    created_at = Column(DateTime(timezone=True), default=datetime.utcnow)
-    updated_at = Column(
-        DateTime(timezone=True), default=datetime.utcnow, onupdate=datetime.utcnow
-    )
-    created_by = Column(String(255), nullable=False)
-
-    package = relationship("Package", back_populates="tags")
-    artifact = relationship("Artifact", back_populates="tags")
-    history = relationship(
-        "TagHistory", back_populates="tag", cascade="all, delete-orphan"
-    )
-
-    __table_args__ = (
-        Index("idx_tags_package_id", "package_id"),
-        Index("idx_tags_artifact_id", "artifact_id"),
-        Index(
-            "idx_tags_package_name", "package_id", "name", unique=True
-        ),  # Composite unique index
-        Index(
-            "idx_tags_package_created_at", "package_id", "created_at"
-        ),  # For recent tags queries
-    )
-
-
-class TagHistory(Base):
-    __tablename__ = "tag_history"
-
-    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
-    tag_id = Column(
-        UUID(as_uuid=True), ForeignKey("tags.id", ondelete="CASCADE"), nullable=False
-    )
-    old_artifact_id = Column(String(64), ForeignKey("artifacts.id"))
-    new_artifact_id = Column(String(64), ForeignKey("artifacts.id"), nullable=False)
-    change_type = Column(String(20), nullable=False, default="update")
-    changed_at = Column(DateTime(timezone=True), default=datetime.utcnow)
-    changed_by = Column(String(255), nullable=False)
-
-    tag = relationship("Tag", back_populates="history")
-
-    __table_args__ = (
-        Index("idx_tag_history_tag_id", "tag_id"),
-        Index("idx_tag_history_changed_at", "changed_at"),
-        CheckConstraint(
-            "change_type IN ('create', 'update', 'delete')", name="check_change_type"
-        ),
-    )
-
-
 class PackageVersion(Base):
     """Immutable version record for a package-artifact relationship.
 
@@ -249,7 +188,7 @@ class Upload(Base):
     artifact_id = Column(String(64), ForeignKey("artifacts.id"), nullable=False)
     package_id = Column(UUID(as_uuid=True), ForeignKey("packages.id"), nullable=False)
     original_name = Column(String(1024))
-    tag_name = Column(String(255))  # Tag assigned during upload
+    version = Column(String(255))  # Version assigned during upload
     user_agent = Column(String(512))  # Client identification
     duration_ms = Column(Integer)  # Upload timing in milliseconds
     deduplicated = Column(Boolean, default=False)  # Whether artifact was deduplicated
@@ -524,8 +463,8 @@ class PackageHistory(Base):
 class ArtifactDependency(Base):
     """Dependency declared by an artifact on another package.
 
-    Each artifact can declare dependencies on other packages, specifying either
-    an exact version or a tag. This enables recursive dependency resolution.
+    Each artifact can declare dependencies on other packages, specifying a version.
+    This enables recursive dependency resolution.
     """
 
     __tablename__ = "artifact_dependencies"
@@ -538,20 +477,13 @@ class ArtifactDependency(Base):
     )
     dependency_project = Column(String(255), nullable=False)
     dependency_package = Column(String(255), nullable=False)
-    version_constraint = Column(String(255), nullable=True)
-    tag_constraint = Column(String(255), nullable=True)
+    version_constraint = Column(String(255), nullable=False)
     created_at = Column(DateTime(timezone=True), default=datetime.utcnow)
 
     # Relationship to the artifact that declares this dependency
     artifact = relationship("Artifact", back_populates="dependencies")
 
     __table_args__ = (
-        # Exactly one of version_constraint or tag_constraint must be set
-        CheckConstraint(
-            "(version_constraint IS NOT NULL AND tag_constraint IS NULL) OR "
-            "(version_constraint IS NULL AND tag_constraint IS NOT NULL)",
-            name="check_constraint_type",
-        ),
         # Each artifact can only depend on a specific project/package once
         Index(
             "idx_artifact_dependencies_artifact_id",