feat(fase-18): migration 027 — list title catalog + per-collective default

Adds `collectives.default_list_title` (nullable text) plus a new
`collective_list_titles (collective_id, title)` table — admins curate via
two SECURITY DEFINER RPCs (`add_list_title` / `remove_list_title`, P0002
on non-admin, idempotent on conflict). Members read; direct writes are
all-deny RLS, matching the item_frequency pattern. Table is added to the
`supabase_realtime` publication with REPLICA IDENTITY FULL so the manage
UI sees admin edits live across browsers.

Numbering rules (#N suffix) stay client-side intentionally — the DB is
agnostic about the format so future "YYYY-MM" or other prefixes don't
require a schema migration. The two-clients-race-on-#6 case is accepted
(documented in migration header); a unique constraint would block
deliberate duplicates.

Wires `default_list_title` through every Collective construction site:
loadUserCollectives in root layout, onboarding, CreateCollectiveModal,
features.ts realtime subscriber, and the manage-collective patcher. The
hand-curated `database.ts` gets the new column, table, and two RPC
signatures.

pgTAP `019_list_title_catalog.sql` (13 assertions): schema invariants on
the column + table, PK shape, RLS posture, two-policies count, RPC
existence + SECURITY DEFINER flag, realtime publication membership, and
the default-NULL behaviour. Role-gate denial is covered by the upcoming
list-title-flow integration suite (postgres bypasses auth.uid inside
pgTAP, same caveat as Fase 15's 018_item_frequency_weight.sql).

Tests: 191 pgTAP (was 178; +13).

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

apps/web/src/routes/onboarding/+page.svelte

@@ -51,7 +51,9 @@
 		const collective = {
 			...(data as { id: string; name: string; emoji: string; created_at: string }),
 			// New collective starts with no flags — every section ON by default.
-			feature_flags: {} as import('@colectivo/types').FeatureFlags
+			feature_flags: {} as import('@colectivo/types').FeatureFlags,
+			// Fase 18: no default list title until an admin sets one.
+			default_list_title: null as string | null
 		};
 
 		userCollectives.update((list) => [...list, collective]);