fix(pgsql): handle nullability/type/default drift on existing columns

Existing databases that already ran an old migration kept stale NOT
NULL constraints and mismatched column types/defaults, because the
schema writer only emitted idempotent ADD COLUMN IF NOT EXISTS guards
and never altered columns that already existed.

- Emit guarded ALTER COLUMN ... SET/DROP NOT NULL when a column's
  nullability differs from the model.
- Emit guarded ALTER COLUMN ... TYPE, falling back to renaming the old
  column and adding a fresh one when the in-place conversion fails.
- Emit guarded ALTER COLUMN ... SET/DROP DEFAULT for default drift.
- Collapse the previously duplicated plain/guarded templates so
  WriteSchema (full-schema, live-state-checking) and WriteMigration
  (diff-based) share the same guarded SQL templates and Go helpers
  instead of maintaining the logic twice.

pkg/writers/pgsql/templates/alter_column_default_with_check.tmpl

@@ -0,0 +1,29 @@
+DO $$
+DECLARE
+  current_default text;
+BEGIN
+  SELECT pg_catalog.pg_get_expr(d.adbin, d.adrelid)
+    INTO current_default
+  FROM pg_attribute a
+  JOIN pg_class t ON t.oid = a.attrelid
+  JOIN pg_namespace n ON n.oid = t.relnamespace
+  LEFT JOIN pg_attrdef d ON d.adrelid = a.attrelid AND d.adnum = a.attnum
+  WHERE n.nspname = '{{.SchemaName}}'
+    AND t.relname = '{{.TableName}}'
+    AND a.attname = '{{.ColumnName}}'
+    AND a.attnum > 0
+    AND NOT a.attisdropped;
+
+{{- if .SetDefault }}
+  IF current_default IS DISTINCT FROM {{quote .DefaultValue}} THEN
+    ALTER TABLE {{qual_table .SchemaName .TableName}}
+      ALTER COLUMN {{quote_ident .ColumnName}} SET DEFAULT {{.DefaultValue}};
+  END IF;
+{{- else }}
+  IF current_default IS NOT NULL THEN
+    ALTER TABLE {{qual_table .SchemaName .TableName}}
+      ALTER COLUMN {{quote_ident .ColumnName}} DROP DEFAULT;
+  END IF;
+{{- end }}
+END;
+$$;