fix: stream SQL database dumps

[Treasure520520]

/claim #59

Summary

This is a focused streaming slice for the large database dump issue.

Instead of building the entire SQL dump in a single in-memory string/blob, /export/dump now returns a ReadableStream and writes the dump incrementally:

• emits the existing SQLite header first
• loads table schemas one table at a time
• reads table rows in bounded LIMIT ? OFFSET ? pages
• streams each batch of INSERT statements as it is produced
• quotes table identifiers and escapes SQL values, including NULL, booleans, bigint, and binary values
• preserves the existing download headers and response shape for callers

This does not attempt to solve the full async/R2 continuation path in one PR. It removes the immediate O(database dump size) string allocation while keeping the existing endpoint behavior intact, so it can compose with later R2/alarm work.

Validation

• ./node_modules/.bin/prettier --check src/export/dump.ts src/export/dump.test.ts
• ./node_modules/.bin/vitest run src/export/dump.test.ts (6 tests)
• git diff --check

The repo-wide ./node_modules/.bin/tsc --noEmit is still blocked by pre-existing type errors outside this PR (src/do.ts, src/operation.ts, existing tests). The focused dump tests and formatting checks pass.

[Treasure520520]

Adding a short demo GIF for the Algora bounty review guideline:

It summarizes the focused change in this PR: stream the SQL dump response, read rows in bounded pages, and avoid a single full-database dump string allocation.