feat(plugins): external replication plugin (pull external -> internal) #72

[FishEnjoyer2025]

Implements pull-based external→internal replication as a plugin, per #72.

What it does

ExternalReplicationPlugin pulls configured tables from the instance's external
data source (e.g. a Supabase Postgres) into the internal Durable-Object SQLite, so
the instance can serve as a close-to-edge read replica that's queried locally instead of
round-tripping to the external database.

• Pull-based — no changes required on the external source (matches the design note
  in the issue: a pull mechanism is a better global solution than per-provider push).
• Per-table config, with optional incremental sync via a cursorColumn
  (updated_at, id, …); the last value is persisted in _starbasedb_replication_state
  so each run only pulls new/changed rows. Omit it to re-pull the whole table.
• Idempotent INSERT OR REPLACE upserts → safe to retry.
• Batched (batchSize, default 5000) so very large tables drain across runs.
• Endpoints: POST /replicate (all tables) and POST /replicate/:table, admin-only.
• Interval scheduling via a Cloudflare Cron Trigger (README includes a
  wrangler.toml example + scheduled() usage).

Tests

8 unit tests (vitest) — passing — covering:

• SELECT generation: full snapshot, incremental first-run (no WHERE), incremental with a
  saved cursor (WHERE + param), resume-from-saved-cursor, and identifier quoting.
• INSERT-OR-REPLACE upsert generation.
• The full replicateTable flow with mocked execution: pull → upsert → advance + persist
  cursor; and the empty-result path.

Files

• plugins/external-replication/index.ts — the plugin
• plugins/external-replication/index.test.ts — tests
• plugins/external-replication/README.md — usage + cron setup

/claim #72