Add package dependencies and project settings page

Package Dependencies:
- Add artifact dependency management system
- Add dependency API endpoints (get, resolve, reverse)
- Add ensure file parsing for declaring dependencies
- Add circular dependency and conflict detection
- Add frontend dependency visualization with graph modal
- Add migration for artifact_dependencies table

Project Settings Page (#65):
- Add dedicated settings page for project admins
- General settings section (description, visibility)
- Access management section (moved from project page)
- Danger zone with inline delete confirmation
- Add Settings button to project page header

backend/app/models.py

@@ -117,6 +117,9 @@ class Artifact(Base):
     tags = relationship("Tag", back_populates="artifact")
     uploads = relationship("Upload", back_populates="artifact")
     versions = relationship("PackageVersion", back_populates="artifact")
+    dependencies = relationship(
+        "ArtifactDependency", back_populates="artifact", cascade="all, delete-orphan"
+    )
 
     @property
     def sha256(self) -> str:
@@ -507,3 +510,54 @@ class PackageHistory(Base):
         Index("idx_package_history_changed_at", "changed_at"),
         Index("idx_package_history_package_changed_at", "package_id", "changed_at"),
     )
+
+
+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.
+    """
+
+    __tablename__ = "artifact_dependencies"
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    artifact_id = Column(
+        String(64),
+        ForeignKey("artifacts.id", ondelete="CASCADE"),
+        nullable=False,
+    )
+    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)
+    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",
+            "artifact_id",
+        ),
+        Index(
+            "idx_artifact_dependencies_target",
+            "dependency_project",
+            "dependency_package",
+        ),
+        Index(
+            "idx_artifact_dependencies_unique",
+            "artifact_id",
+            "dependency_project",
+            "dependency_package",
+            unique=True,
+        ),
+    )