feat(sprint-1): implement /join, guided 7-step onboarding, and next-action dashboard

[ashishsinghbora]

Sprint 1 Summary

• Dedicated /join page: High-converting entry point emphasizing real open-source work and proof of work over certificates with live platform metrics.
• Fresh DB Authorization: Added src/lib/auth/authorization.ts with getFreshAuthenticatedUser to protect privileged endpoints from stale JWT role claims.
• 7-Step Guided Onboarding: Step-by-step wizard capturing experience level, languages, technologies, developer tools, technical interests, contribution preferences, time commitment, and primary goals with live matching issue preview upon completion.
• Next-Action Dashboard: Streamlined contributor dashboard leading with the active/recommended next issue contribution and step-by-step milestones.
• Schema Migration: Added drizzle/0001_sprint_lifecycle_tables.sql covering issue_claims, pull_requests, pull_request_reviews, github_webhook_deliveries, notifications, and mentor_requests.

All tests passing (36/36), types check, linting clean, build succeeded.