fix(export): stream large database dumps with keyset pagination (#59)

[trongtruong110-ux]

Problem

dumpDatabaseRoute builds the entire dump — every row of every table — into a single in-memory string before responding (let dumpContent = '' … then new Blob([dumpContent])). For a large database this exceeds the isolate's memory limit and the request fails outright, which is what #59 reports.

Changes

Rewrites the SQL dump path to stream instead of buffer:

• Bounded memory. Rows are read one page at a time and the response body is a pull-driven ReadableStream. The runtime only asks for the next page once the previous chunk has flushed downstream, so peak memory is ~one page regardless of database size.
• Keyset pagination (O(n)). Pages use WHERE _rowid_ > ? ORDER BY _rowid_ LIMIT ? rather than LIMIT/OFFSET. OFFSET re-scans every skipped row on each page (O(n²)), which is unusable on a large table. WITHOUT ROWID tables (which have no _rowid_) fall back to LIMIT/OFFSET.
• Correctness. Table identifiers are quoted; values are encoded as proper SQL literals — NULL, numbers, 0/1 for booleans, X'..' for blobs, and escaped strings. Internal sqlite_* tables are skipped.
• A database/connection error that occurs before streaming starts still returns a clean 500; the table list is resolved eagerly for that reason.

No route or public API change — dumpDatabaseRoute(dataSource, config) is unchanged (an optional pageSize argument is added for tests and tuning).

Tests

src/export/dump.test.ts is rewritten to cover: streamed output with headers, multi-page keyset pagination (asserting the cursor query is used and OFFSET is not), value encoding (NULL / number / boolean / blob / quote-escaping), the WITHOUT ROWID fallback, an empty database, and the 500 error path.

Follow-up

This fixes the memory blow-up for the synchronous /export/dump endpoint. For databases large enough to also exceed the Worker CPU-time limit within a single request, a follow-up can offload the dump to R2 via a Durable Object alarm. Happy to do that as a separate PR.

/claim #59