Adds the §4.5 Auto-compaction section to architecture.md and
architecture.es.md describing the trigger, fallback, persistence and
the new SSE 'compaction' event so consumers know how to react.
Includes the three placeholder projects used while exercising the
feature end to end (bot-onboarding, dashboard-metricas, tienda-ropa)
so /api/reindex picks them up without further setup.
