bregctl dev: webhook and PostGIS modes so tutorials stop cloning the repository

Found in the 0.28.0 pre-release review (2026-09-08). Line references are as of main after #926.

`bregctl dev` covers the plain registry case with a pinned PostgreSQL image (`crates/registry-bregctl/src/dev/mod.rs:44-45`). Two tutorials still clone the repository because dev has no equivalent: `send-registry-events-to-a-webhook.mdx` needs a webhook receiver, and `query-a-spatial-registry-from-qgis.mdx` needs a PostGIS image.

Proposed: a receiver mode (or a documented pairing with a local receiver) and a PostGIS image option on `bregctl dev`, each with a test, so both tutorials can be rebuilt on `bregctl init` plus `bregctl dev` and the quickstart directory can be retired.

Start at crates/registry-bregctl/src/dev/mod.rs:44-45 to understand the existing PostgreSQL setup, then read send-registry-events-to-a-webhook.mdx and query-a-spatial-registry-from-qgis.mdx. Define the receiver pairing and PostGIS image option, add a test for each, and verify both tutorials work with bregctl init plus bregctl dev so the quickstart directory is no longer needed.

Written by the indexing model from the issue text.