feat(fase-9): migration 015 — users.theme column for dark mode

Adds `theme text not null default 'system' check (theme in
('light','dark','system'))` to public.users. New users get 'system' so
the UI follows the OS preference until they opt in; existing rows
backfill via the default.

Includes pgTAP test 009 (8 assertions covering column existence/type,
default value, valid transitions, and the check constraint rejecting
unknown values). Wires the theme field into the hand-curated
database.ts type so PostgREST queries stay typed.

Also hardens `just db-types` to bail out (instead of truncating
database.ts) when the supabase CLI is missing — the recipe was silently
wiping the file on this dev machine.

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

supabase/tests/009_user_theme.sql

@@ -0,0 +1,69 @@
+-- pgTAP: public.users.theme column (migration 015)
+-- Run with: psql -U postgres -d postgres -f supabase/tests/009_user_theme.sql
+--
+-- Fase 9.1 — dark mode feature flagship. Adds `theme` to public.users with
+-- three valid values: 'light' | 'dark' | 'system'. New users default to
+-- 'system' so the UI matches the OS preference until they opt in.
+
+CREATE EXTENSION IF NOT EXISTS pgtap;
+
+BEGIN;
+SELECT plan(8);
+
+-- ── Column existence + type ─────────────────────────────────────────────
+SELECT has_column(
+    'public', 'users', 'theme',
+    'public.users has a theme column'
+);
+
+SELECT col_type_is(
+    'public', 'users', 'theme', 'text',
+    'public.users.theme is of type text'
+);
+
+SELECT col_not_null(
+    'public', 'users', 'theme',
+    'public.users.theme is NOT NULL'
+);
+
+SELECT col_default_is(
+    'public', 'users', 'theme', 'system',
+    'public.users.theme defaults to system'
+);
+
+-- ── Check constraint behaviour ──────────────────────────────────────────
+-- Use Ana (seed admin) so the row exists.
+UPDATE public.users SET theme = 'light'
+WHERE id = '11111111-1111-1111-1111-111111111111';
+SELECT is(
+    (SELECT theme FROM public.users WHERE id = '11111111-1111-1111-1111-111111111111'),
+    'light',
+    'theme can be set to light'
+);
+
+UPDATE public.users SET theme = 'dark'
+WHERE id = '11111111-1111-1111-1111-111111111111';
+SELECT is(
+    (SELECT theme FROM public.users WHERE id = '11111111-1111-1111-1111-111111111111'),
+    'dark',
+    'theme can be set to dark'
+);
+
+UPDATE public.users SET theme = 'system'
+WHERE id = '11111111-1111-1111-1111-111111111111';
+SELECT is(
+    (SELECT theme FROM public.users WHERE id = '11111111-1111-1111-1111-111111111111'),
+    'system',
+    'theme can be set to system'
+);
+
+-- Invalid value must be rejected by the check constraint.
+SELECT throws_ok(
+    $$UPDATE public.users SET theme = 'sepia' WHERE id = '11111111-1111-1111-1111-111111111111'$$,
+    '23514',
+    NULL,
+    'theme rejects values other than light|dark|system'
+);
+
+SELECT * FROM finish();
+ROLLBACK;