fix: stream and async R2 database dumps

[akamabu]

/claim #59

Summary

Fixes #59 by moving database dumps away from building the entire dump in memory and adding an R2-backed asynchronous export path for large databases.

What changed

• /export/dump now returns a ReadableStream instead of assembling a single giant string/blob in memory.
• Table data is exported in bounded LIMIT/OFFSET pages (EXPORT_CHUNK_SIZE configurable, default 500).
• SQL identifiers and values are safely escaped for generated INSERT statements.
• Added optional async mode: GET /export/dump?async=true
  • Requires EXPORT_BUCKET R2 binding.
  • Writes dump_YYYYMMDD-HHMMSSZ.sql to R2 by default.
  • Supports filename= query parameter.
  • Supports callback notifications via callbackUrl= or EXPORT_CALLBACK_URL.
  • Uses ctx.waitUntil when available so the request can return 202 Accepted while R2 upload continues.
• Added Env/config wiring for EXPORT_BUCKET, EXPORT_CALLBACK_URL, and EXPORT_CHUNK_SIZE.
• Expanded dump tests for streaming, pagination, escaping, async R2 upload, custom filename, and callback behavior.

Testing

• npx vitest run src/export/dump.test.ts ✅
• npx vitest run was also run; the new dump tests pass, while 4 pre-existing unrelated RLS tests fail in src/rls/index.test.ts.