feat(db): add user.onboardingStep for wizard resume-state

Persists the highest step a user reached in the rebuilt /onboarding flow
so closing the tab mid-flow no longer wipes progress. Nullable Int
default 0; encoding mirrors the new wizard:
  0 welcome / 1 goals / 2 source / 3 baseline / 4 done

Migration is idempotent (ADD COLUMN IF NOT EXISTS) so it tolerates
hand-applied dev-DB state without diverging from the migration history.

prisma/migrations/0057_user_onboarding_step/migration.sql

@@ -0,0 +1,26 @@
+-- v1.4.25 W14b — onboarding wizard resume-state.
+--
+-- Adds `onboarding_step` to the `users` table so the rebuilt multi-step
+-- wizard at `/onboarding/[step]` can resume the user at the highest
+-- step they reached. Without this column, closing the tab mid-flow
+-- wipes progress and forces users back to step 0.
+--
+-- Step encoding (mirrors `src/components/onboarding/OnboardingShell.tsx`
+-- and `src/app/onboarding/[step]/page.tsx`):
+--   0 = welcome    (carousel / value-prop intro)
+--   1 = goals      ("what do you want to track?")
+--   2 = source     ("connect a source" — Withings / Apple Health / manual)
+--   3 = baseline   (first measurement / sync confirmation)
+--   4 = done       (success screen; same write that flips
+--                   `onboarding_completed_at` from NULL to NOW())
+--
+-- Default 0 keeps the column non-null in practice for every new user,
+-- while existing pre-W14b rows stay NULL (because the `DEFAULT 0`
+-- only applies to *new* inserts on PostgreSQL — existing rows are not
+-- backfilled). Wizard mount code treats NULL as 0.
+--
+-- The `IF NOT EXISTS` guards make the migration idempotent — if the
+-- column landed via a prior dev-DB hand-edit, the migration still
+-- runs cleanly to completion.
+ALTER TABLE "users"
+  ADD COLUMN IF NOT EXISTS "onboarding_step" INTEGER DEFAULT 0;

prisma/schema.prisma

@@ -87,6 +87,18 @@ model User {
 
   // Onboarding
   onboardingCompletedAt DateTime? @map("onboarding_completed_at")
+  // v1.4.25 W14b — last persisted onboarding wizard step.
+  // Encodes resume-from-where-you-left-off semantics for the rebuilt
+  // multi-step wizard at `/onboarding/[step]`. Values mirror the
+  // wizard's step indices:
+  //   0 = welcome
+  //   1 = goals
+  //   2 = source
+  //   3 = baseline
+  //   4 = done (triggers `onboardingCompletedAt` flip on the same
+  //             round-trip — the column stays at 4 for completed users)
+  // Null = legacy pre-W14b user; the wizard treats null as 0 on mount.
+  onboardingStep Int? @default(0) @map("onboarding_step")
   // v1.4.15 Phase B5: tracks whether the user has finished (or
   // explicitly dismissed) the spotlight tour that overlays the
   // dashboard on first arrival. Distinct from the wizard at