feat(pgsql): unique constraints on apply, matching unique index names, FK deferral

- migration writer now emits unique constraints (direct apply and live diff
  previously dropped them); idempotent via the existing DO block
- unique constraint adopts a same-named model unique index with
  ADD CONSTRAINT ... UNIQUE USING INDEX instead of building a second index
- dbml reader names unnamed unique indexes ukey_<table>_<cols> to match the
  unique constraint naming
- foreign key deferral: Constraint.Deferrable/InitiallyDeferred are *bool
  (nil = unspecified, never diffed); writers emit DEFERRABLE, DEFERRABLE
  INITIALLY DEFERRED and NOT DEFERRABLE; live reader reads condeferrable and
  condeferred; dbml reader/writer support deferrable and initially deferred
- tests and README/naming docs updated

pkg/writers/pgsql/NAMING_CONVENTIONS.md

@@ -8,8 +8,8 @@ Standardized naming rules for all database objects in RelSpec PostgreSQL output.
 | ----------------- | ----------- | ---------------------------------- | ------------------------ |
 | Primary Key       | `pk_`       | `pk_<schema>_<table>`              | `pk_public_users`        |
 | Foreign Key       | `fk_`       | `fk_<table>_<referenced_table>`    | `fk_posts_users`         |
-| Unique Constraint | `uk_`       | `uk_<table>_<column>`              | `uk_users_email`         |
-| Unique Index      | `uidx_`     | `uidx_<table>_<column>`            | `uidx_users_email`       |
+| Unique Constraint | `ukey_`     | `ukey_<table>_<column>`            | `ukey_users_email`       |
+| Unique Index      | `ukey_`     | `ukey_<table>_<column>`            | `ukey_users_email`       |
 | Regular Index     | `idx_`      | `idx_<table>_<column>`             | `idx_posts_user_id`      |
 | Check Constraint  | `chk_`      | `chk_<table>_<constraint_purpose>` | `chk_users_age_positive` |
 | Sequence          | `identity_` | `identity_<table>_<column>`        | `identity_users_id`      |
@@ -43,25 +43,25 @@ Standardized naming rules for all database objects in RelSpec PostgreSQL output.
 
 ### Unique Constraints
 
-**Pattern:** `uk_<table>_<column>`
+**Pattern:** `ukey_<table>_<column>`
 
-- Use `uk_` prefix strictly for database constraints (CONSTRAINT type)
+- Generated by the DBML reader for column `[unique]`
 - Include column name for clarity
 - Examples:
-  - `uk_users_email`
-  - `uk_users_username`
-  - `uk_products_sku`
+  - `ukey_users_email`
+  - `ukey_users_username`
+  - `ukey_products_sku`
 
 ### Unique Indexes
 
-**Pattern:** `uidx_<table>_<column>`
+**Pattern:** `ukey_<table>_<column>`
 
-- Use `uidx_` prefix strictly for index type objects
-- Distinguished from constraints for clarity and implementation flexibility
+- Unnamed unique indexes use the same name as a unique constraint on the same columns
+- Writers then build the constraint on the index (`ADD CONSTRAINT ... UNIQUE USING INDEX`), so no duplicate index is created
 - Examples:
-  - `uidx_users_email`
-  - `uidx_sessions_token`
-  - `uidx_api_keys_key`
+  - `ukey_users_email`
+  - `ukey_sessions_token`
+  - `ukey_api_keys_key`
 
 ### Regular Indexes