Added Github API rate limiting

- Implemented comprehensive GitHub API rate limit handling:
    - Integrated @octokit/plugin-throttling for automatic retry with exponential backoff
    - Added RateLimitManager service to track and enforce rate limits
    - Store rate limit status in database for persistence across restarts
    - Automatic pause and resume when limits are exceeded
    - Proper user identification for 5000 req/hr authenticated limit (vs 60 unauthenticated)

  - Improved rate limit UI/UX:
    - Removed intrusive rate limit card from dashboard
    - Toast notifications only at critical thresholds (80% and 100% usage)
    - All rate limit events logged for debugging

  - Optimized for GitHub's API constraints:
    - Reduced default batch size from 10 to 5 repositories
    - Added documentation about GitHub's 100 concurrent request limit
    - Better handling of repositories with many issues/PRs

src/lib/utils/concurrency.ts

@@ -10,7 +10,7 @@
 export async function processInParallel<T, R>(
   items: T[],
   processItem: (item: T) => Promise<R>,
-  concurrencyLimit: number = 5,
+  concurrencyLimit: number = 5, // Safe default for GitHub API (max 100 concurrent, but 5-10 recommended)
   onProgress?: (completed: number, total: number, result?: R) => void
 ): Promise<R[]> {
   const results: R[] = [];