perf(storage): skip the slug recompute query on non-slug connection updates

[pedrofrxncx]

Follows #6240, which stopped ConnectionStorage.update() decrypting secrets for the slug-merge lookup but still ran that lookup (and the slug recompute) unconditionally on every call.

Most calls to update() never touch app_name/connection_url/title — the fields the slug is derived from. The hottest one is on the proxy request path (apps/api/src/api/routes/proxy.ts), which calls update(connectionId, { status: "active" | "error" }) on connection recovery/failure and was paying an extra unnecessary SELECT round-trip for a slug that can't have changed.

This adds a presence check ("app_name" in data || ...) before running the lookup+recompute, so a pure status/heartbeat write skips it entirely. Behavior is unchanged: any update that does touch a slug-source field still goes through the exact same lookup/merge/recompute path as before.

How to confirm: read apps/api/src/storage/connection.ts update() — the recompute block is now gated on touchesSlugField, and serializeConnection still only writes fields present in data, so a status-only update still doesn't touch slug.

Local checks: bun run fmt, bunx tsc --noEmit (apps/api, clean), bunx oxlint apps/api/src/storage/connection.ts (0 warnings/errors). No unit test exists for this path (only Postgres-backed integration tests in connection.integration.test.ts, not runnable here) — full CI validates those.

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Summary by cubic

Skip slug lookup and recompute in ConnectionStorage.update() when the update does not touch slug-source fields, removing an extra SELECT on hot status/heartbeat writes. Previously update() always fetched app_name, connection_url, and title to recompute the slug; now it does so only if any of those keys are present in data.

Behavior for slug-affecting updates is unchanged: the same lookup/merge/recompute path runs, and serializeConnection still only persists provided fields, so status-only updates leave slug untouched. This targets the proxy path in apps/api/src/api/routes/proxy.ts that calls update(connectionId, { status: ... }).

^{Written for commit 2292c28. Summary will update on new commits.}