feat(core): integrate TAPI backoff v2 into upload flow

Integrates backoff infrastructure and error classification into the
SegmentDestination upload flow. This completes the TAPI backoff v2
implementation.

SegmentDestination changes:
- Upload gate: Check canUpload() before processing batches
- Batch lifecycle: Create metadata on first failure only
- Error handling:
  - 429 rate limit: Call handle429() and halt upload loop
  - Transient errors: Create/update batch metadata for retry
  - Permanent errors: Drop batch immediately
  - Network errors: Treat as transient
- Retry count header: Send X-Retry-Count (batch or global)
- Dynamic import: Lazy load backoff module to avoid circular deps
- Clean initialization: Info-level logs for normal operations
- Error logging: All catch blocks now log errors appropriately

API changes:
- Add keepalive: true to fetch for background completion
- Add retryCount parameter for X-Retry-Count header

Logger changes:
- Default to disabled in production (process.env.NODE_ENV check)

Key improvements from code review:
- No console.log statements
- Simplified upload gate logic
- Proper error logging throughout
- Appropriate log levels (info for normal, error for failures)

This PR depends on PRs #1 (infrastructure) and #2 (error classification).

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: abueide

packages/core/src/api.ts

@@ -4,21 +4,28 @@ export const uploadEvents = async ({
   writeKey,
   url,
   events,
+  retryCount = 0,
 }: {
   writeKey: string;
   url: string;
   events: SegmentEvent[];
-}) => {
+  retryCount?: number;
+}): Promise<Response> => {
+  // Create Authorization header (Basic auth format)
+  const authHeader = 'Basic ' + btoa(writeKey + ':');
+
   return await fetch(url, {
     method: 'POST',
     keepalive: true,
     body: JSON.stringify({
       batch: events,
       sentAt: new Date().toISOString(),
-      writeKey: writeKey,
+      writeKey: writeKey, // Keep in body for backwards compatibility
     }),
     headers: {
       'Content-Type': 'application/json; charset=utf-8',
+      'Authorization': authHeader,
+      'X-Retry-Count': retryCount.toString(),
     },
   });
 };