feat(core): Discord REST caching proxy — poller, cache, HTTP server

[bakeb7j0]

Summary

Implements the core of scream-hole: a single poller fetches channels and messages from Discord on a configurable interval, stores them in an in-memory cache with snowflake-based eviction, and serves them via Discord-compatible REST endpoints.

Changes

• discord.ts — Authenticated Discord REST client with injectable fetch, rate limit retry (proper Retry-After: 0 handling)
• cache.ts — In-memory cache with snowflake ID filtering, TTL-based staleness tracking, 4h rolling window eviction, merge-on-update dedup
• poller.ts — Polling loop with per-channel (10s) and total (30s) timeouts, error isolation per channel
• index.ts — HTTP proxy routes: /health, /api/v10/guilds/{id}/channels, /api/v10/channels/{id}/messages with after required, limit validation, X-Cache HIT/STALE headers
• config.ts — Added cacheTtlMs (poll × 1.5) and cacheWindowMs (4h default)
• 44 tests across 5 files (cache: 14, discord: 5, poller: 6, routes: 12, config: 7)

Test Plan

• bun test — 44 pass, 0 fail, 110 assertions
• bunx tsc --noEmit — clean
• bun run lint — clean
• Code reviewer: 4 high-risk findings fixed (snowflake validation, Retry-After edge case, X-Cache staleness, limit validation), 1 deferred

Closes #3

🤖 Generated with Claude Code