fix(query): auto-coerce Postgres ENUM columns to text on read (#26)

[turnerrainer]

Summary

Fixes #26. A bare SELECT status FROM t where status is a Postgres user-defined enum previously returned JSON null for the enum column, because sqlx's built-in String decoder isn't compatible with the enum's dynamic OID and every branch of pg_column_value fell through to the last-resort text probe (which also errors on unknown OIDs).

The row materialiser now inspects PgTypeInfo::kind() and, for PgTypeKind::Enum (and PgTypeKind::Array(Enum)), decodes via row.try_get_raw + <String as Decode<Postgres>>::decode directly. Postgres transmits enum values as UTF-8 labels in both text and binary formats, so no cast is required.

• Author's preferred option (Option 1 in the issue).
• No caller-visible break — the wire shape becomes what a hand-written ::text cast would have produced.

Schema changes

• New Liquibase changeset 010-enum-table — ticket_priority enum + tickets table with a scalar enum column and an enum-array column.
• New test fixture fixtures-tickets — three rows exercising the scalar, mixed-array, and empty-array cases.

Existing operator DBs are unaffected: Liquibase is idempotent by changeset ID and the new changeset only adds a type + a table.

Test plan

• cargo +1.88.0 fmt --all -- --check
• cargo clippy --all-targets --locked -- -D warnings
• SQLite-only suite (no Postgres) — 313 tests pass
• Postgres integration suite — 67 tests pass (4 new)
  • pg_enum_column_selected_without_cast_returns_label_string
  • pg_enum_column_multi_row_labels_are_string
  • pg_enum_array_column_returns_array_of_labels
  • pg_empty_enum_array_returns_empty_array

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