storage: replace the postgres contract blob with a columnar layout

[bennyhodl]

Summary

Replaces the opaque contract blob in the Postgres store with one row per contract in a new dlc_contracts table, and migrates existing databases in place. Old rows keep loading, the store moves them on startup, and it warns while any are left. Closes #190.

Changes

• New dlc_contracts table (migration 0010). The offer, accept, and sign wire messages are stored byte for byte, so the TLV streams from messages: keep the TLV records appended to an offer instead of dropping them #188 travel inside them on every state with no suffix and no per-state table. The manager-only state that the messages do not carry has one typed column per field. A format_version on each row guards the next layout change. The down migration refuses to run while the table holds rows.
• One reader and one writer, ContractRow::into_contract and ContractRow::from_contract, in ddk/src/storage/postgres/contract_row.rs. Every read path of the store goes through them. The metadata comes from the wrapped offered contract, because the Contract accessors return zeros for a closed contract and unwrap its CET.
• Legacy path in ddk/src/storage/postgres/legacy.rs. PostgresStore::migrate_legacy_contracts moves each blob into a row, one transaction per contract, and reports each row it cannot move instead of stopping. PostgresStore::new runs it when migrations are on. A row still in the old layout loads through a fallback on every read path and moves on its next update. The metadata queries union the old table for rows not yet moved.
• Warnings at startup and on every legacy read while any contract is left, naming the command to run.
• ddk-node --postgres-url <url> migrate runs the schema migrations and the move on its own, prints the count and every failed id, and exits non-zero if anything is left.
• OfferedContract::keys_id() getter, since the field is crate private and the row needs it.
• Operator doc at docs/postgres-contract-migration.md, linked from the README. The follow-up to drop the legacy tables is storage: drop the legacy contract_data and contract_metadata tables #207.

Testing

• Row round trip is byte for byte for every fixture, including the pre-marker offered blob, and for the wrapped states: rejected, refunded, failed accept, failed sign. TLV records on all three layers of a closed contract survive the row. An unknown row version and a missing column both error and name the problem.
• Postgres: migration at startup moves the blobs and empties the legacy tables; a legacy row loads through every read path with migrations off and moves on its first update; an undecodable blob is reported and does not stop startup; TLV records on a seeded legacy blob come through the migration; a contract closed by refund writes null CET and attestation columns and reads back; the offers list holds only offers we received.
• Ignored live check legacy_blobs_round_trip_against_a_live_database reads every blob in a real database and compares bytes without writing. Run against a dlcd regtest database with 11 contracts: all 11 match. The same database migrated at dlcd startup with migrated=11 failed=0, and the dlcd CLI reads (get-contracts, get-contract, get-status, get-messages, get-dlc-txns) return correct data for accepted and confirmed contracts.
• Clippy is clean for the changed files, including the store tests.