chore: hard migrate encryption owner DID column

[skgbafa]

Summary

• remove the SeaORM compatibility mapping from encryption_network.owner_did to the old principal column
• add a forward migration that renames the existing encryption_network principal column to owner_did
• keep final runtime/model/schema surfaces on owner_did with no model-level legacy alias

Validation

• cargo fmt
• cargo test -p tinycloud-core encryption_network
• cargo check -p tinycloud-node
• git diff --check

Notes

The only remaining principal strings are in the migration history, where they identify the old column being renamed.