feat(activity): per-tenant audit feed in notification dropdown

activity_events records booking lifecycle transitions with the actor
behind each change — staff (dashboard), customer (self-service), or
system (auto-promotion). The notification dropdown leads with the real
feed instead of derived recent-booking guesses.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/migrations/00016_activity_events.sql

@@ -0,0 +1,21 @@
+-- +goose Up
+-- Per-tenant activity feed: who changed what. Actor kinds distinguish
+-- dashboard users (staff), public self-service actions (customer), and
+-- automatic transitions (system). Kept to booking lifecycle events in v1.
+CREATE TABLE activity_events (
+  id            uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  tenant_id     uuid NOT NULL REFERENCES tenants(id) ON DELETE CASCADE,
+  actor_kind    text NOT NULL DEFAULT 'staff',
+  actor_subject text NOT NULL DEFAULT '',
+  actor_name    text NOT NULL DEFAULT '',
+  action        text NOT NULL,
+  entity_type   text NOT NULL DEFAULT 'booking',
+  entity_id     uuid,
+  summary       text NOT NULL DEFAULT '',
+  created_at    timestamptz NOT NULL DEFAULT now()
+);
+CREATE INDEX activity_events_tenant_created_idx
+  ON activity_events (tenant_id, created_at DESC);
+
+-- +goose Down
+DROP TABLE IF EXISTS activity_events;