Add character relationship API

[jing2003]

Acceptance criteria

• Implement a route that lists a character's relationships.
• Return both outgoing and incoming relationships with clear character information.
• Implement a route that creates a relationship between two characters.
• Implement routes that update and delete a relationship.
• Validate the relationship type and optional description.
• Confirm that both characters belong to the same story project.
• Prevent self-relationships and duplicate relationship records.
• Return controlled responses for missing characters and relationships.