storage: drop the legacy contract_data and contract_metadata tables

[bennyhodl]

Note

🤖 Claude Fable 5.1 responding on behalf of bennyhodl

Context

#190 moved contracts into the columnar dlc_contracts table. The two legacy tables, contract_data and contract_metadata, stay as the version one layout so that a database written by a release up to 2.0 keeps loading. The store moves every blob into a row on startup, warns while any blob is left, and ddk-node migrate runs the move by hand. See docs/postgres-contract-migration.md.

What to do, in a later release

1. Remove the version one reader and the fallback reads in ddk/src/storage/postgres/legacy.rs.
2. Remove the legacy union from the metadata queries in ddk/src/storage/postgres/mod.rs.
3. Add a schema migration that drops contract_data and contract_metadata. The migration must refuse to run while either table still holds a row, so that no contract is stranded.
4. Remove ContractData from ddk/src/storage/sqlx.rs.
5. Say in the release notes that the release needs a database that was opened at least once by the release that shipped storage: replace the opaque postgres contract blob with a versioned, column-based schema #190, or that ddk-node migrate was run.

Acceptance

• A database with no legacy rows upgrades cleanly.
• A database with legacy rows fails the migration with a message that names ddk-node migrate.