fix: support DEFERRABLE attribute on unique constraints

[joaquinhuigomez]

Read condeferrable and condeferred from pg_constraint during schema introspection. Include deferrable state in constraint comparison and DDL generation.

Previously, deferrable unique constraints always produced a spurious diff because the tool didn't track the deferrable attribute. When recreating the constraint via ADD CONSTRAINT ... UNIQUE USING INDEX, the DEFERRABLE / INITIALLY DEFERRED clause was omitted, so the regenerated schema never matched the original.

Changes:

• queries.sql / queries.sql.go: fetch condeferrable and condeferred from pg_constraint in the GetIndexes query
• schema.go: add IsDeferrable and IsInitiallyDeferred fields to IndexConstraint
• sql_generator.go: append DEFERRABLE INITIALLY DEFERRED or DEFERRABLE INITIALLY IMMEDIATE to the ADD CONSTRAINT ... USING INDEX DDL when applicable
• index_cases_test.go: acceptance tests for adding, no-op, and changing deferrable unique constraints

Closes #273

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