feat: support batch/bulk REST operations

[MajorTal]

Problem

Approving 12 pending members = 12 individual PATCH requests. CSV import of 200 members = 200 POSTs. No way to batch operations.

Suggested Solution

Support PostgREST's bulk insert (POST with array body) and document it. Or add a batch endpoint that accepts an array of operations.

Context

Discovered during Wild Lychee admin member management. Workaround: use an edge function with db.sql() for bulk updates.