docs: stack architecture diagrams for Apollo / ImageApi / FVR

Mermaid sources for the system context, data ownership, photo ingest,
watcher tick, Apollo photo matching and photo insight chat, traced from
the current code of all three repos. Allow-lists docs/architecture/
alongside docs/adr/ and docs/agents/.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitignore

@@ -11,6 +11,7 @@ database/target
 /docs/*
 !/docs/agents/
 !/docs/adr/
+!/docs/architecture/
 /specs
 
 # Default ignored files