feat: add hourly hb_agent_runtime_v1 usage events for Coder Agent runtime (#27312)

closes CODAGT-839
closes CODAGT-843
closes CODAGT-773

## Summary

Adds a new heartbeat usage event type, `hb_agent_runtime_v1`, measuring
the total agent-loop runtime of Coder Agents (chats) per UTC hour, plus
a reconciler that generates one event per hour with self-healing
backfill over a trailing 7-day window. Events flow to Tallyman through
the existing publisher unchanged. This measures the new Coder Agents
(the `chats` tables), not the deprecated Tasks counted by
`dc_managed_agents_v1`.

Independent of #27508, which fixes the dead ai-seats cron registration.
Both PRs carry the identical `usage_event` create permission hunk for
the usage-publisher subject (this feature's generator and the ai-seats
cron each need it for heartbeat inserts), so they can land in either
order and the overlap merges cleanly.

> [!WARNING]
> **Do not include this in a release until Tallyman accepts
`hb_agent_runtime_v1`.** The publisher marks permanently rejected events
as done-forever, and the generator then sees those buckets as complete
locally, so their usage would be silently and permanently lost.

## Details

Each event's payload is `{"runtime_ms": N}`: the sum of
`chat_messages.runtime_ms` for messages created in the hour bucket `[H,
H+1)`, across all chats (sub-agents, API-created, archived, and
soft-deleted messages included). Events use deterministic IDs
(`hb_agent_runtime_v1:<bucket start>`) with `created_at` set to the
bucket start, so concurrent replicas race safely via `ON CONFLICT (id)
DO NOTHING` without locking, and daily rollups attribute backfilled
hours to the correct day. Idle hours produce zero-valued events. A
bucket becomes eligible 5 minutes after it closes; hours missing for
longer than the 7-day window are forfeited, which can only undercount.

Note that this makes `usage_events.created_at` explicitly the *event
occurrence time* rather than the row insertion time; the two only
diverge for backfilled events. It already behaved as the occurrence
timestamp (it drives the daily rollup day and is shipped to
Tallyman/Metronome as the event timestamp), and the migration now
documents this with a `COMMENT ON COLUMN`, which also surfaces as a Go
doc comment on `UsageEvent.CreatedAt`.

The new `usage.Generator` runs unconditionally in enterprise builds; the
`publish_usage_data` license flag continues to gate egress only, so
air-gapped deployments still fill their local ledger. The
`aggregate_usage_event()` trigger sums `runtime_ms` per day into
`usage_events_daily` (unlike `hb_ai_seats_v1`, which takes the daily
max).

`InsertHeartbeatUsageEvent` now takes an explicit `createdAt` so
generators can backfill historical buckets; the cron passes
`clock.Now()` to preserve its existing behavior.

## Tallyman follow-up

<details>
<summary>Prompt for the Tallyman-repo agent</summary>

> **Task**: Add support for the new Coder usage event type
`hb_agent_runtime_v1` so Tallyman accepts, validates, and forwards it to
Metronome.
>
> **Background**: coder/coder PR (this PR) adds hourly heartbeat events
measuring Coder Agent runtime. Events arrive via the existing
`/api/v1/events/ingest` endpoint with: `event_type:
"hb_agent_runtime_v1"`, `event_data: {"runtime_ms": <int64 >= 0>}`,
deterministic `id` of the form `hb_agent_runtime_v1:2026-07-15_14:00:00`
(UTC hour bucket start), and `created_at` set to the bucket start (may
be up to ~8 days in the past due to backfill; within Metronome's 34-day
dedup window). Zero-value events are normal (idle hours).
>
> **Work**:
> 1. Update Tallyman's vendored/imported `coderd/usage/usagetypes` (or
equivalent) to the coder/coder commit that adds
`UsageEventTypeHBAgentRuntimeV1` and `HBAgentRuntime`.
> 2. Ensure ingestion validation accepts the type (`Valid()` switches)
and rejects negative `runtime_ms`.
> 3. Ensure Metronome forwarding maps the event with transaction ID
derived from the event `id` as for existing types, passing `runtime_ms`
through as the property for a SUM-aggregated billable metric ("Coder
Agent Hours" = `SUM(runtime_ms) / 3,600,000`).
> 4. Do NOT permanently reject unknown-but-well-formed future `hb_*`
types if avoidable; at minimum confirm current behavior for unknown
types (temporary vs permanent rejection) and report it.
> 5. Tests: ingest accept/validate, dedup by ID, Metronome payload
mapping.
>
> **Constraint**: this must be deployed to tallyman-prod **before** any
coder/coder release containing the event generator; coderd treats
permanent rejections as terminal per event.

</details>

coderd/database/dump.sql

@@ -728,7 +728,7 @@ CREATE FUNCTION aggregate_usage_event() RETURNS trigger
     AS $$
 BEGIN
     -- Check for supported event types and throw error for unknown types.
-    IF NEW.event_type NOT IN ('dc_managed_agents_v1', 'hb_ai_seats_v1') THEN
+    IF NEW.event_type NOT IN ('dc_managed_agents_v1', 'hb_ai_seats_v1', 'hb_agent_runtime_v1') THEN
         RAISE EXCEPTION 'Unhandled usage event type in aggregate_usage_event: %', NEW.event_type;
     END IF;
 
@@ -756,6 +756,13 @@ BEGIN
 						COALESCE((NEW.event_data->>'count')::bigint, 0)
 					)
 				)
+            -- Hourly runtime heartbeats: sum the runtime per day.
+            WHEN NEW.event_type IN ('hb_agent_runtime_v1') THEN
+                jsonb_build_object(
+                    'runtime_ms',
+                    COALESCE((usage_events_daily.usage_data->>'runtime_ms')::bigint, 0) +
+                    COALESCE((NEW.event_data->>'runtime_ms')::bigint, 0)
+                )
         END;
 
     RETURN NEW;
@@ -3529,7 +3536,7 @@ CREATE TABLE usage_events (
     publish_started_at timestamp with time zone,
     published_at timestamp with time zone,
     failure_message text,
-    CONSTRAINT usage_event_type_check CHECK ((event_type = ANY (ARRAY['dc_managed_agents_v1'::text, 'hb_ai_seats_v1'::text])))
+    CONSTRAINT usage_event_type_check CHECK ((event_type = ANY (ARRAY['dc_managed_agents_v1'::text, 'hb_ai_seats_v1'::text, 'hb_agent_runtime_v1'::text])))
 );
 
 COMMENT ON TABLE usage_events IS 'usage_events contains usage data that is collected from the product and potentially shipped to the usage collector service.';
@@ -3540,6 +3547,8 @@ COMMENT ON COLUMN usage_events.event_type IS 'The usage event type with version.
 
 COMMENT ON COLUMN usage_events.event_data IS 'Event payload. Determined by the matching usage struct for this event type.';
 
+COMMENT ON COLUMN usage_events.created_at IS 'The time the usage occurred, which is not necessarily the time the row was inserted. Events that measure a time bucket (e.g. hb_agent_runtime_v1) always set this to the bucket start, regardless of when the row was inserted. This timestamp determines the day used by the daily rollup trigger and is sent to the usage collector service as the event timestamp.';
+
 COMMENT ON COLUMN usage_events.publish_started_at IS 'Set to a timestamp while the event is being published by a Coder replica to the usage collector service. Used to avoid duplicate publishes by multiple replicas. Timestamps older than 1 hour are considered expired.';
 
 COMMENT ON COLUMN usage_events.published_at IS 'Set to a timestamp when the event is successfully (or permanently unsuccessfully) published to the usage collector service. If set, the event should never be attempted to be published again.';
@@ -4870,6 +4879,8 @@ CREATE INDEX idx_template_versions_has_ai_task ON template_versions USING btree
 
 CREATE UNIQUE INDEX idx_unique_preset_name ON template_version_presets USING btree (name, template_version_id);
 
+CREATE INDEX idx_usage_events_agent_runtime ON usage_events USING btree (event_type, created_at) WHERE (event_type = 'hb_agent_runtime_v1'::text);
+
 CREATE INDEX idx_usage_events_ai_seats ON usage_events USING btree (event_type, created_at) WHERE (event_type = 'hb_ai_seats_v1'::text);
 
 CREATE INDEX idx_usage_events_select_for_publishing ON usage_events USING btree (published_at, publish_started_at, created_at);