feat(export): implement chunked R2 database export with DO alarm cont…

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Purpose

Implement chunked R2-based database export to support any database size (up to 10GB).
Uses DO alarms for exports exceeding the 30-second Worker timeout, with breathing
intervals to prevent database lock contention. Includes callback URL notifications
and new status/download endpoints.

/claim #59

Tasks

• Replace in-memory dump with batched R2 chunked export (1000 rows/batch)
• Implement DO alarm continuation for long-running exports
• Add breathing intervals (2s) between alarm cycles
• Add POST /export/dump with optional callbackUrl param
• Add GET /export/status/:exportId endpoint
• Add GET /export/download/:exportId endpoint
• Add R2 bucket binding to wrangler.toml
• Backwards compatible fallback when R2 is not configured
• Comprehensive test suite (15 tests)

Verify

1. Run pnpm vitest run src/export/chunked-dump.test.ts — 15 tests should pass
2. Run pnpm vitest run — all 166 existing tests still pass
3. Configure R2 bucket, deploy, and hit GET /export/dump on a small DB — should return .sql file directly
4. Hit POST /export/dump?callbackUrl=https://example.com on a large DB — should return 202 with exportId
5. Poll GET /export/status/:exportId — should show progress
6. After completion, GET /export/download/:exportId — should return the .sql file

Before

After

#Short Demo video of changes

Recording.2026-03-02.201444.1.mp4

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Implementation Summary

Problem

The current GET /export/dump endpoint loads the entire database into memory before returning it as a response. This fails for:

• Databases approaching the 1GB Durable Object limit (10GB in future)
• Any export that takes longer than 30 seconds (Worker timeout)

Solution

Chunked R2-based export with DO alarm continuation:

• Batched processing: Reads rows in batches of 1000 with a 20s time limit per cycle
• R2 storage: Streams SQL chunks to R2 instead of holding everything in memory
• DO alarms: When approaching the 30s timeout, saves progress to DO storage and schedules an alarm to resume after a 2s breathing interval
• Dual delivery: Small exports (<20s) return directly; large exports return 202 with an exportId for polling via /export/status/:exportId and downloading via /export/download/:exportId
• Callback URL: Optional ?callbackUrl= parameter sends a POST notification when export completes or fails
• Backwards compatible: Falls back to the original in-memory dump when R2 is not configured

New Endpoints

Method	Path	Description
GET	/export/dump	Enhanced dump (R2 chunked if available, else legacy)
POST	/export/dump?callbackUrl=...	Start async export with callback
GET	/export/status/:exportId	Check export progress
GET	/export/download/:exportId	Download completed export from R2

Tests

15 new tests covering initialization, chunk processing, NULL handling, error handling, callbacks, status endpoint, and download endpoint. All existing tests unaffected.

/claim #59