Update enum PostgreSQL mapping to use native ENUM type

PostgreSQL supports native ENUM via CREATE TYPE ... AS ENUM, which
provides similar semantics to MySQL ENUM (efficient storage, value
enforcement, definition-order ordering). DataJoint will handle the
separate type creation automatically.

Co-authored-by: dimitri-yatsenko <dimitri@datajoint.com>

Author: claude

docs/src/design/tables/storage-types-spec.md

@@ -103,7 +103,7 @@ for serialized Python objects.
 |-----------|-------------|-------|------------|
 | `json` | JSON document | `JSON` | `JSONB` |
 | `uuid` | UUID | `BINARY(16)` | `UUID` |
-| `enum(...)` | Enumeration | `ENUM(...)` | `VARCHAR` + check |
+| `enum(...)` | Enumeration | `ENUM(...)` | `CREATE TYPE ... AS ENUM` |
 
 ### Native Passthrough Types