Stream SQL, CSV, and JSON exports with paged reads

[BobbyZ2026]

Purpose

Fixes #59
/claim #59

This refactors the export endpoints so large SQLite exports do not have to materialize the full response in Durable Object memory before returning.

Changes

• Adds shared streaming export helpers that return ReadableStream responses instead of Blob-backed full strings.
• Pages table reads with a bounded default page size and cooperative runtime yields between full pages.
• Uses keyset pagination on primary keys or rowid cursors, with OFFSET only as a fallback when cursor values are unavailable.
• Quotes SQLite identifiers, serializes SQL values safely, preserves CSV escaping, and keeps rowid cursor aliases out of exported rows.
• Updates SQL dump, CSV, and JSON routes plus regression tests for streaming helpers and route behavior.

Verification

• npx pnpm@10.12.1 exec vitest run src/export/dump.test.ts src/export/csv.test.ts src/export/json.test.ts src/export/streaming.test.ts
• npx pnpm@10.12.1 run build

Full-suite note: npx pnpm@10.12.1 exec vitest run still has four pre-existing RLS failures in src/rls/index.test.ts that are unrelated to this export change.