Feat/rate limiting

[Yentec]

Description

Upstash Redis rate limiting for createPost and toggleVote

Changes

• Installed @upstash/ratelimit and @upstash/redis
• Created lib/ratelimit.ts — two named limiters (slidingWindow for createPost, fixedWindow for toggleVote), fallback to no-op if Upstash is not configured
• Integrated into server/actions/posts.ts and server/actions/votes.ts — check after auth(), before any Prisma request
• Added UPSTASH_REDIS_REST_URL and UPSTASH_REDIS_REST_TOKEN to .env.example
• Updated CLAUDE.md — corrected the architecture tree, rate limiting spec, and Server Actions flow

Validation

• Unit tests pass (26/26)
• Clean typecheck
• Build successful
• Rate limiters tested with mock vi.hoisted + success: false case
• Dev server functional without Upstash credentials

Closes #13