FAS 1b: Progressive onboarding (3-layer flow)

- Add PROGRESSIVE_ONBOARDING feature flag to KNOWN_FLAGS
- Add onboarding_step column to users table (a/b/c)
- New backend endpoints:
  - GET /v1/onboarding/status – check current step + feature flag
  - POST /v1/onboarding/quick-start – complete Step A (goal + precision)
  - POST /v1/onboarding/complete-b – complete Step B (diet, allergens, household)
  - POST /v1/onboarding/complete-c – complete Step C (health profile)
  - POST /v1/onboarding/skip – GDPR-friendly skip
- Refactor mobile onboarding screen into 3 progressive layers
- Update auth store with onboardingStep state
- Update tab layout to only block on Step A
- Update registration to set onboardingStep='a'
- Add i18n keys for Step B/C titles across all 12 locales
- Add Zod validation schemas (quickStartInputSchema, onboardingStatusSchema)
- Add tests for validation and feature flag
- Preserve existing /v1/me/onboarding for backward compatibility
- Migration: 0004_progressive_onboarding.sql

infrastructure/migrations/0004_progressive_onboarding.sql

@@ -0,0 +1,8 @@
+-- FAS 1b: Progressive onboarding (3-layer flow)
+-- Step A = immediate value, Step B = after first value, Step C = contextual/completed
+ALTER TABLE "users" ADD COLUMN "onboarding_step" text NOT NULL DEFAULT 'a';
+
+-- Add feature flag for progressive onboarding rollout
+INSERT INTO "feature_flags" ("key", "enabled", "rollout_percent", "description_sv")
+VALUES ('progressive_onboarding', false, 0, 'Progressiv onboarding: 3-stegs flow (FAS 1b)')
+ON CONFLICT ("key") DO NOTHING;