fix: enforce uniqueness and hour alignment for agent runtime usage events (#27983)

The usage generator writes `hb_agent_runtime_v1` rows with `created_at`
at the UTC hourly bucket start and exactly one row per bucket, but
nothing in the schema enforced either invariant. A duplicate bucket row
under a different id would be double-counted by any consumer summing
`runtime_ms`, and a misaligned `created_at` would skew which usage
period a bucket is attributed to.

This replaces the non-unique partial index
`idx_usage_events_agent_runtime` (from migration 000561) with a unique
index of the same shape and adds an hour-alignment `CHECK` constraint.
Both statements validate existing rows: every supported writer has
always produced conforming data, so a pre-existing violator is anomalous
and failing the migration loudly beats silently rewriting usage rows.
`generateBucket` treats a unique violation on the bucket index as
another replica having won the race, mirroring the existing `ON CONFLICT
(id)` no-op for committed rows.

The `coderd/notifications` sync commit and its revert cancel out (the
drift they addressed was fixed on main by #27979); the PR's net diff is
only the usage-event changes.

Part 1 of a 3-PR stack splitting up #27796 (see there for review
history). Stack: this PR → #27984 → #27985.

coderd/database/migrations/000570_usage_events_agent_runtime_invariants.down.sql

@@ -0,0 +1,9 @@
+-- IF EXISTS tolerates the index already being gone (e.g. rolling back out
+-- of order during an incident) instead of failing.
+DROP INDEX IF EXISTS idx_usage_events_agent_runtime;
+CREATE INDEX idx_usage_events_agent_runtime
+  ON usage_events (event_type, created_at)
+  WHERE event_type = 'hb_agent_runtime_v1';
+
+ALTER TABLE usage_events
+  DROP CONSTRAINT IF EXISTS usage_events_agent_runtime_hour_aligned;

coderd/database/migrations/000570_usage_events_agent_runtime_invariants.up.sql

@@ -0,0 +1,24 @@
+-- The usage generator writes hb_agent_runtime_v1 rows with created_at at
+-- the UTC hourly bucket start and exactly one row per bucket. Uniqueness
+-- keeps any consumer that sums runtime_ms from counting a bucket twice;
+-- the alignment CHECK protects the attribution model, which charges a
+-- bucket to the usage period containing its start.
+--
+-- Both statements validate existing rows. Every supported writer has always
+-- produced conforming data, so a pre-existing violator is anomalous and
+-- failing the migration loudly beats silently rewriting usage rows.
+ALTER TABLE usage_events
+  ADD CONSTRAINT usage_events_agent_runtime_hour_aligned
+  CHECK (
+    event_type <> 'hb_agent_runtime_v1'
+    OR date_trunc('hour', (created_at AT TIME ZONE 'UTC')) = (created_at AT TIME ZONE 'UTC')
+  );
+
+-- Inserts keep their (id) arbiter: re-inserting a bucket under its
+-- deterministic id stays a silent no-op, while a duplicate bucket row under
+-- a different id raises a unique violation (generateBucket in
+-- enterprise/coderd/usage/generator.go handles it).
+DROP INDEX idx_usage_events_agent_runtime;
+CREATE UNIQUE INDEX idx_usage_events_agent_runtime
+  ON usage_events (event_type, created_at)
+  WHERE event_type = 'hb_agent_runtime_v1';