feat(plugins): add replication plugin for external database sync (#72)

[voldemort9999]

/claim #72

Description

Adds a lightweight replication plugin that automatically pulls data from a configured external database (PostgreSQL, MySQL, Turso) into the internal local SQLite database.

This implementation deeply integrates with the existing architecture and alarm mechanism without altering the core StarbaseDBDurableObject system:

• Implements plugins/replication/index.ts cleanly within ~110 lines.
• Extends the existing executeExternalQuery mapping logic with INSERT OR REPLACE INTO natively to replicate rows idempotently.
• Utilizes the cursorColumn (default: id) and state tracking via an automatically provisioned tmp_replication_cursors SQLite table to ensure duplicate data isn't fetched and replication scales independently per table.
• Relies cleanly on the existing application ecosystem structure, avoiding unnecessary code bloat or intrusive dependencies across do.ts or route handlers.

Local Testing Performed

• Validated external source (PostgreSQL) SELECT batching via dynamically mapped cursors.
• Successfully verified edge cases when external results return empty batches (cursor tracks cleanly, upsert logic exits gracefully).
• Validated full vitest pass on local logic simulating large schema syncs to ensure robust state management during the mapping step.
• Checked TypeScript compiler to assure zero type defects or regressions.

Demo Video

os.01.mp4