Fix migration, relational model and metadata for "sprechrollen"

[YourMJK]

Code:

• New "sprechrolleTeil" table in relational model for referencing Sprechrolle in parent
• New "position" column in "spricht" table for maintaining ordering of Sprecher
• Fixed migration of "sprechrollen" for Hörspiel with "teile" and added checks to ensure data is well-formed

Metadata:

• Fixed and adapted "sprechrollen" for migration of:
  • Spezial "und der dreiTag"
  • Kurzgeschichten

Database:

• Performed and exported the updated migration