feat: implement streaming export for large databases

[769066112-ops]

Summary

Fixes #59 — Database dumps do not work on large databases.

Problem

All export endpoints (/export/dump, /export/csv/:table, /export/json/:table) load the entire dataset into memory before creating the response. For large databases (up to 10GB on Durable Objects), this causes:

• Memory overflow (OOM)
• Request timeouts (30-second Cloudflare Workers limit)
• Partial or failed exports

Solution

Replace in-memory buffering with ReadableStream-based streaming combined with paginated database queries (LIMIT/OFFSET, 1000 rows per page).

Changes

src/export/index.ts — New streaming utilities:

• forEachPage() — Paginated query helper that fetches rows in batches via LIMIT/OFFSET and invokes a callback per page
• tableExists() — Lightweight table existence check
• createStreamingExportResponse() — Creates a chunked-transfer Response from a ReadableStream
• Original getTableData() and createExportResponse() preserved for backward compatibility

src/export/dump.ts — Streaming SQL dump:

• Streams schema + INSERT statements per table
• Each table's rows are fetched page-by-page, never holding the full dataset in memory

src/export/csv.ts — Streaming CSV export:

• Streams header row + data rows with proper CSV escaping
• Extracted escapeCsvValue() helper for cleaner escaping logic

src/export/json.ts — Streaming JSON export:

• Streams a valid JSON array incrementally ([, row, ,\n, row, ... ])
• Each page of rows is serialized and enqueued immediately

Design Decisions

• Page size of 1000 rows — Balances memory usage vs. DB round-trips
• LIMIT/OFFSET pagination — Simple, works with any SQLite table, no cursor/rowid dependency
• No external dependencies — Uses Web Streams API (native to Cloudflare Workers)
• Backward compatible — Original helper functions retained; no changes to route signatures in handler.ts

Testing

The streaming approach can be verified by:

1. Creating a large test table (e.g., 1M+ rows)
2. Hitting /export/dump, /export/csv/:table, /export/json/:table
3. Confirming the response streams back without timeout or OOM

---

Bounty: $250 · Issue #59