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/migrations/015_user_theme.sql

@@ -0,0 +1,16 @@
+-- Migration 015: theme preference on public.users (Fase 9.1)
+--
+-- Stores the user's chosen UI theme: 'light' | 'dark' | 'system'.
+-- New users (and existing rows on upgrade) default to 'system' so the UI
+-- inherits the OS preference until they explicitly opt in.
+--
+-- The JWT minted by GoTrue caches `auth.users` (not `public.users`), so
+-- adding this column does NOT invalidate existing sessions — clients read
+-- the value via PostgREST `SELECT theme FROM users WHERE id = auth.uid()`.
+
+ALTER TABLE public.users
+    ADD COLUMN theme text NOT NULL DEFAULT 'system'
+    CHECK (theme IN ('light', 'dark', 'system'));
+
+COMMENT ON COLUMN public.users.theme IS
+    'UI theme preference. ''system'' follows prefers-color-scheme.';