fix: support schema privilege diffs

[dilame]

What

Plan and apply schema-level authorization drift:

• schema privileges (GRANT / REVOKE on schemas), including WITH GRANT OPTION changes;
• schema ownership (ALTER SCHEMA ... OWNER TO ...) when target owner differs;
• CREATE SCHEMA ... AUTHORIZATION ... for newly materialized schemas so validation can faithfully rebuild non-default owners.

This adds schema ACLs and schema owners to the introspected NamedSchema model. Owner-granted USAGE / CREATE ACL entries are treated as implicit owner privileges rather than ordinary grant drift.

Why

pg-schema-diff already tracks table privileges, but schema authorization state was incomplete. Before this PR, a migration plan could converge to zero statements while the source and target still differed on schema grants.

After the first schema-privilege fix, one production-shaped no-op remained: schemas owned by a role can expose ACL rows like service=UC/service. PostgreSQL owners already have implicit schema privileges, so those owner grants should not create repeated no-op GRANT USAGE ON SCHEMA ... TO service noise.

A second production-shaped drift then surfaced: the tool read schema owner names only to suppress implicit owner grants, but did not generate ALTER OWNER. This PR now treats schema ownership as real schema state, so ownership drift is caught by the plan instead of requiring manual repair SQL.

How

• internal/queries/queries.sql: add GetSchemaPrivileges from pg_namespace.nspacl; make GetSchemas return schema owner names.
• internal/schema/schema.go: add SchemaPrivilege, attach sorted privileges to NamedSchema, and keep schema owner for ownership diffs plus implicit-owner filtering.
• pkg/diff/sql_generator.go: diff schema privileges, recreate when WITH GRANT OPTION changes, and filter old privileges by old owner / target privileges by target owner.
• pkg/diff/schema_privilege_sql_generator.go: emit GRANT ... ON SCHEMA / REVOKE ... ON SCHEMA with AUTHZ_UPDATE hazards.
• pkg/diff/named_schema_sql_generator.go: emit owner-aware CREATE SCHEMA ... AUTHORIZATION ... and ALTER SCHEMA ... OWNER TO ... with AUTHZ_UPDATE hazard.
• pkg/diff/plan_generator.go: clear schema privileges during validation for skipped privilege statements, matching table privilege handling.
• internal/migration_acceptance_tests/named_schema_cases_test.go: cover grant, revoke, grant-option drift, non-owner CREATE revocation, owner-grant no-op, and explicit owner drift.

Test

PATH="/Applications/Postgres.app/Contents/Versions/16/bin:$PATH" go test ./internal/migration_acceptance_tests -run TestNamedSchemaTestCases -count=1
PATH="/Applications/Postgres.app/Contents/Versions/16/bin:$PATH" go test ./pkg/diff -count=1
PATH="/Applications/Postgres.app/Contents/Versions/16/bin:$PATH" go test ./internal/schema -count=1
PATH="/Applications/Postgres.app/Contents/Versions/16/bin:$PATH" go test ./internal/migration_acceptance_tests -run TestMaterializedViewTestCases/'Add materialized view with dependent column' -count=1

Also attempted broader runs:

PATH="/Applications/Postgres.app/Contents/Versions/16/bin:$PATH" go test ./...
PATH="/Applications/Postgres.app/Contents/Versions/16/bin:$PATH" go test $(go list ./... | grep -v '^github.com/stripe/pg-schema-diff$') -count=1
PATH="/Applications/Postgres.app/Contents/Versions/16/bin:$PATH" go test ./internal/migration_acceptance_tests -count=1

The root go test ./... is blocked in this local worktree by an untracked scratch test_diff.go at repository root. The broader acceptance package hit local temp-database statement-timeout noise in unrelated tests; the named-schema suite and the isolated materialized-view case passed when rerun directly.