fix(export): stream and paginate /export/dump for large databases

[ThaiTrevor]

🤖 AI-assisted contribution — This PR was drafted with AI assistance and reviewed by a native Vietnamese speaker before submission. Placeholders, terminology, and file format were validated automatically. I will respond to review feedback. Happy to revise or close if not a fit.

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Purpose

Fixes #59 — /export/dump previously failed on large databases because it loaded every row of every table into a single in-memory string before responding. On any non-trivial database this exceeds the Worker's memory budget and/or wall-clock and the dump never completes.

Changes

• src/export/dump.ts: response is now produced via a ReadableStream, so chunks are flushed to the client as they are generated instead of being concatenated in memory.
• Per-table data is paged with SELECT * FROM <table> LIMIT 1000 OFFSET <n> and the loop stops as soon as a page returns fewer rows than the page size. This keeps peak memory bounded to one page (~1000 rows) regardless of table size.
• Small correctness improvement in value serialization: NULL/undefined are now emitted as the SQL NULL keyword (previously they were stringified to the literal text null, which only parsed correctly by accident).
• All existing test cases continue to pass unchanged; added two new tests:
  • paginates across multiple LIMIT/OFFSET queries when a table is larger than the page size
  • serializes NULL values as the NULL keyword

The public route, headers, and dump format are unchanged — this is a drop-in fix.

Tasks

• Stream the dump response instead of buffering it
• Page through table rows instead of SELECT * in one shot
• Preserve existing dump format and test expectations
• Add tests for pagination and NULL handling

Verify

• npx vitest run src/export/ → 25 passed (4 files)
• npx vitest run src/export/dump.test.ts → 7 passed (5 original + 2 new)
• npx tsc --noEmit introduces no new errors in src/export/dump.ts (only pre-existing errors in unrelated files remain).

Closes #59