docs: use native PostgreSQL enum type instead of CHECK constraint

PostgreSQL enum implementation:
- Adapter creates named type: {schema}_{table}_{column}_enum
- Column references the created type
- Type lifecycle managed automatically (create/drop with table)

Both MySQL and PostgreSQL now use native enum types.

Author: claude

docs/design/multi-backend-plan.md

@@ -68,10 +68,22 @@ with the understanding that these are not portable to PostgreSQL.
 
 `enum` remains a core type with backend-specific implementation:
 - **MySQL:** Native `ENUM('a','b','c')` inline in column definition
-- **PostgreSQL:** `VARCHAR(n)` with `CHECK (column IN ('a','b','c'))` constraint
+- **PostgreSQL:** Native enum type via `CREATE TYPE` + column reference
 
-The adapter automatically translates enum definitions to the appropriate backend syntax.
-This provides portable enum semantics while using native capabilities where available.
+PostgreSQL enum implementation:
+```sql
+-- Adapter creates named type before table creation
+CREATE TYPE {schema}_{table}_{column}_enum AS ENUM ('a', 'b', 'c');
+
+-- Column references the type
+CREATE TABLE {table} (
+    {column} {schema}_{table}_{column}_enum NOT NULL
+);
+
+-- Type is dropped when table is dropped (via CASCADE or explicit cleanup)
+```
+
+The adapter handles type lifecycle automatically (creation, cleanup on table drop).
 
 ### 1.3 Final Core Types
 
@@ -118,7 +130,7 @@ CORE_TYPES = {
 | Decision | Rationale |
 |----------|-----------|
 | **No unsigned integers** | PostgreSQL has no unsigned types; cannot be portably emulated |
-| **Enum via CHECK on PostgreSQL** | PostgreSQL native enum requires separate `CREATE TYPE`; CHECK constraint provides equivalent validation |
+| **Enum via CREATE TYPE on PostgreSQL** | Adapter manages enum type lifecycle (create before table, drop with table) |
 | **No text type** | `varchar(n)` sufficient for most uses; native `text` available per-backend |
 | **datetime (not timestamp)** | Matches Python naming; familiar to scientists; avoids MySQL/PG timestamp confusion |
 | **json → jsonb on PostgreSQL** | Better performance and indexing; key order not guaranteed by JSON spec anyway |
@@ -613,7 +625,7 @@ Test against both databases in GitHub Actions with Python 3.9-3.12.
 | `char(n)` | `char(n)` | `char(n)` | |
 | `varchar(n)` | `varchar(n)` | `varchar(n)` | |
 | `decimal(p,s)` | `decimal(p,s)` | `numeric(p,s)` | |
-| `enum(...)` | `enum(...)` | `varchar` + CHECK | CHECK constraint emulates enum |
+| `enum(...)` | `enum(...)` | `CREATE TYPE` + reference | Native enum on both backends |
 
 ## Appendix B: Native Types (Backend-Specific)