feat(fase-20): migration 028 — language_code ENUM gains 'eu'

Extends public.language_code with the Euskera (Basque, ISO 639-1) label.
Plan §20.1 was written against a CHECK-constraint mental model; the
actual gate is the ENUM from migration 001, so the right primitive is
ALTER TYPE … ADD VALUE. End-user contract is identical — 'eu' accepted,
'fr' rejected — and rejection surfaces as the canonical 22P02
invalid_text_representation error.

pgTAP `supabase/tests/020_users_language_eu.sql` (5 assertions, LANG-EU-
T01..T05) pins both ends: ENUM membership and UPDATE round-trip on Ana.

Hand-curated database.ts widened to `'en' | 'es' | 'eu'`. The supabase
CLI isn't on this host, so db-types regen is the manual edit per the
recipe's fallback path.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

supabase/migrations/028_users_language_eu.sql

@@ -0,0 +1,21 @@
+-- Migration 028: add Euskera (Basque, ISO 639-1 `eu`) to the supported
+-- `public.language_code` ENUM (Fase 20).
+--
+-- The plan (`plan/fase-20-euskera-locale.md` §20.1) was written against the
+-- assumption that `public.users.language` is gated by a CHECK constraint.
+-- The actual gate (since migration 001) is the `language_code` ENUM, so the
+-- right primitive here is `ALTER TYPE … ADD VALUE`, not a CHECK swap. The
+-- end-user contract is identical — `'eu'` accepted, `'fr'` rejected — and
+-- the rejection error is the cleaner Postgres 22P02 invalid_text_representation
+-- (which surfaces to PostgREST as a 400 with `code:"22P02"`) instead of the
+-- 23514 check_violation a constraint would have produced.
+--
+-- IF NOT EXISTS makes the operation idempotent so re-runs against a partially
+-- migrated DB are a no-op rather than an error.
+--
+-- ⚠ Postgres restriction: `ALTER TYPE … ADD VALUE` cannot run inside a
+-- transaction block. The migration runner applies each .sql file
+-- atomically per-file, so the statement must stand alone. Do NOT wrap it
+-- in BEGIN/COMMIT.
+
+ALTER TYPE public.language_code ADD VALUE IF NOT EXISTS 'eu';