Stream large database exports

[mdp28]

/claim #59

Summary

• Streams CSV, JSON, and SQL dump exports instead of materializing the full export payload in memory before responding.
• Adds shared export streaming helpers for paginated table reads, runtime yielding, CSV/SQL value formatting, safe filenames, and ReadableStream response creation.
• Keeps export routes responsive for large tables by reading rows in pages and yielding chunks progressively.
• Adds focused coverage for streaming helpers plus CSV/JSON/dump route behavior.

Verification

• corepack pnpm install --frozen-lockfile --ignore-scripts
• corepack pnpm exec vitest --run src/export
• git diff --check

Targeted test result: 5 files passed, 34 tests passed.