feat: add the growthsignals activity taxonomy and emitter - #6057

GRW-66

First of a five-PR stack. Nothing calls this package yet, so no events are emitted and no behaviour changes.

Summary

Adds server/internal/growthsignals/, which describes notable moments in Gram as a single PostHog gram_activity event with a stable property shape:

• The activity taxonomy, and the map from audit action to activity.
• The event builder, which produces the event name, the distinct id, and the property map.
• A repo-backed enricher behind a TTL cache, resolving organization, project and actor ids into the names a Slack reader needs.
• The emitter, which skips the demo organization and logs capture failures rather than returning them.

Also includes the design document the rest of the stack follows.

Two decisions worth a reviewer's attention:

• Uncurated audit actions pass through under a normalized name rather than being dropped, so the firehose channel has coverage automatically and there is no allowlist to keep extending. A small exclusion list keeps genuinely high-volume noise out.
• dashboard_url is never omitted. A Slack destination that renders it as a button link fails the whole message when the url is empty, so it falls back to the organization's page and then to the site root.

Motivation

The events Growth wants to see do not exist in PostHog. There is no event for a project being created, an MCP server deployed, a security policy written, or a member joining an existing organization.

Every one of those mutations is already recorded by the audit logger and published to Pub/Sub, and gram streams already consumes that stream. The signal exists; nothing forwards it. This package is the shared vocabulary the rest of the stack uses to do that.

Temporal actions/month: 0, scales with fixed. No background work is added.

Summary by cubic

Adds the growthsignals package, which describes notable moments in Gram as a single PostHog gram_activity event with a stable property shape. This is PR 1 of a five-PR stack for GRW-66 (revamping PostHog Slack notifications); nothing calls the package yet, so no events are emitted and no behavior changes.

The package carries the activity taxonomy, the map from audit action to activity, the event builder, a repo-backed enricher behind a TTL cache, and the emitter, which skips the demo organization and logs capture failures rather than returning them. The design document the rest of the stack follows is included.

Reviewer notes

• Uncurated audit actions pass through under a normalized name rather than being dropped, so the firehose has automatic coverage with no allowlist to maintain; a small exclusion list keeps genuinely high-volume noise out.
• dashboard_url is never omitted: a Slack destination that renders it as a button link fails the whole message when the URL is empty, so it falls back to the organization's page and then to the site root. With no site URL configured, it is omitted rather than sent blank, so the template can drop the button instead of failing the message.
• Per-activity extras cannot occupy reserved property keys, so an activity can never rewrite the event's identity.
• Concurrent cache misses for the same key collapse into a single lookup that re-checks the cache before loading. The shared load runs detached from the caller that started it, so one caller's cancellation leaves without failing the query for everyone else waiting; a burst of events from one organization stays at one query.

^{Written for commit 2a19984. Summary will update on new commits.}