document_mode for Meilisearch sink

PR #2025 changed the Meilisearch client from PUT (add or update) to POST (add or replace). This makes sense as the default — users expect documents to be fully replaced.

However, for multi-table CDC (multiple Postgres tables → one Meilisearch index), PUT partial merge is essential. Each sink sends only its table's columns, and Meilisearch merges them at document level. With POST, each sink overwrites the entire document, erasing other sinks' data.

This adds a document_mode config option:

• :replace (default) — POST, same behavior as today
• :update — PUT, partial merge for multi-table CDC

YAML config example

sinks:
  - name: meilisearch-auctions
    destination:
      type: meilisearch
      document_mode: update  # PUT partial merge

AUTH_DISABLED env var

Self-hosted deployments behind reverse proxies (Cloudflare Access, Authelia, Authentik, etc.) don't need Sequin's built-in login — the proxy handles authentication. Setting AUTH_DISABLED=true auto-logs in the default provisioned user, skipping the login page entirely.

Self-hosted build fixes

• config/prod.exs — treat empty SENTRY_DSN as nil (crashes Sentry at startup)
• Dockerfile — skip mix sentry.package_source_code when SELF_HOSTED=1
• .gitignore — ignore sst-env.d.ts (auto-generated by SST in monorepo builds)

Changes