feat: Add Cloudflare Workers observability and logging

[paveg]

Summary

• Enable Cloudflare Workers Logs with full request sampling in wrangler.toml
• Add Analytics Engine integration (devcard_metrics dataset) for metrics collection
• Create new observability.ts utility with structured logging and W3C Trace Context support
• Enhance logger middleware to record metrics and add trace headers to responses
• Add GitHub API monitoring with rate limit tracking and warnings

Changes

wrangler.toml

• Add [observability] section with logs enabled
• Add [[analytics_engine_datasets]] for metrics storage
• Add LOG_LEVEL environment variable
• Add nodejs_compat compatibility flag

New File: src/utils/observability.ts

• Logger class with structured JSON logging (debug/info/warn/error levels)
• Observability class for Analytics Engine integration
• Request, error, cache, and GitHub API metrics recording
• W3C Trace Context utilities (traceId/spanId generation)

Enhanced: src/middleware/logger.ts

• W3C Trace Context support (traceparent header parsing)
• Response headers: X-Trace-Id, X-Span-Id, X-Ray-Id
• Automatic log level based on status code (4xx=warn, 5xx=error)
• Analytics Engine integration for request metrics

Enhanced: src/fetchers/github.ts

• API call timing and metrics recording
• Rate limit monitoring with warnings when remaining < 100
• Endpoint-specific tracking (user_stats, top_languages, repo_info)

Test plan

• Run pnpm typecheck - passes
• Run pnpm build - passes
• Run npx wrangler deploy --dry-run - shows correct bindings
• Deploy to Cloudflare and verify logs appear in dashboard
• Verify Analytics Engine dataset receives metrics

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