feat(onboarding): add multi-step user onboarding wizard

Add a 5-step onboarding flow for new users:
- Migration 017: adds onboarding_completed, onboarding_completed_at,
  contact_email, preferred_destination to user_profiles
- app/api/onboarding.py: REST endpoints (status, profile, plan, storage,
  complete) with session-based auth using sub/preferred_username/email/id
  priority chain
- app/views/onboarding.py: GET /onboarding view with configured-destination
  detection helper for all 8 supported storage providers
- frontend/templates/onboarding.html: Alpine.js wizard with progress
  indicator, tier cards (server-rendered), storage destination cards,
  accessible markup (WCAG AA), and all fetch() API calls
- app/auth.py: redirect first-time OAuth users (onboarding_completed=False)
  to /onboarding after login
- 16 unit tests covering all endpoints, auth enforcement, and edge cases

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

app/api/__init__.py

@@ -15,6 +15,7 @@ from app.api.duplicates import router as duplicates_router
 from app.api.files import router as files_router
 from app.api.google_drive import router as google_drive_router
 from app.api.logs import router as logs_router
+from app.api.onboarding import router as onboarding_router
 from app.api.onedrive import router as onedrive_router
 from app.api.openai import router as openai_router
 from app.api.plans import router as plans_router
@@ -60,3 +61,4 @@ router.include_router(webhooks_router)
 router.include_router(database_router)
 router.include_router(subscriptions_router)
 router.include_router(plans_router)
+router.include_router(onboarding_router)