Stream database dump exports

[akmittal006]

Fixes #59

/claim #59

Purpose

Stream SQL database dump responses so large exports no longer build a single in-memory SQL string before returning a file. This directly addresses the current /export/dump memory pressure path while keeping the response-download behavior intact for bounded requests.

Tasks

• Replace full SQL dump string accumulation with a ReadableStream response
• Page table reads in 500-row batches instead of loading each table at once
• Quote SQLite identifiers for schema lookups, SELECTs, and generated INSERT statements
• Preserve SQL value escaping for strings, booleans, nulls, and binary values
• Add regression coverage for batched export behavior

Verify

• pnpm exec vitest run src/export/dump.test.ts src/export/index.test.ts src/export/csv.test.ts src/export/json.test.ts
• pnpm exec prettier --check src/export/dump.ts src/export/dump.test.ts
• git diff --check

Before

/export/dump accumulated the full dump in memory by appending table DDL and every row INSERT to one string before wrapping it in a Blob. Large databases could exhaust memory or block too long before a response could be streamed back.

After

/export/dump returns a ReadableStream, writes the SQLite header and table schema incrementally, and fetches row INSERTs in bounded batches with short yields between full batches.