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/readers/dbml/README.md

@@ -86,16 +86,40 @@ Ref: posts.user_id > users.id [delete: cascade]
 - Table definitions with columns
 - Primary keys (`pk`)
 - Not null constraints (`not null`)
-- Unique constraints (`unique`)
+- Unique constraints (`unique`) → constraint `ukey_<table>_<col>`
 - Default values (`default`)
 - Inline references (`ref`)
 - Standalone `Ref` blocks
+- FK deferral: `deferrable`, `initially deferred` (see below)
 - Commented cross-file refs (`// Ref:` — see below)
 - Indexes and composite indexes
 - Table notes and column notes
 - Enums
 - Dialect directives (`@postgres:` / `@sqlite:` — see below)
 
+## Naming
+
+| Source | Generated name |
+| ------ | -------------- |
+| Column `[unique]` | constraint `ukey_<table>_<col>` |
+| Unnamed unique index | index `ukey_<table>_<cols>` (same name as the constraint on the same columns) |
+| Unnamed index | `idx_<table>_<cols>` |
+
+Explicit `name:` is never changed. Leading `_` in the table name is stripped.
+
+## FK deferral
+
+```
+col int [ref: > t.id, deferrable, initially deferred]
+Ref: a.x > b.id [delete: cascade, deferrable]
+```
+
+| Setting | SQL |
+| ------- | --- |
+| `deferrable` | `DEFERRABLE` |
+| `initially deferred` | `DEFERRABLE INITIALLY DEFERRED` |
+| neither | unspecified (existing FK left as is) |
+
 ## Commented cross-file refs
 
 `// Ref:` / `// ref:` lines (ignored by dbdiagram) become FKs + relationships once both ends are loaded.