docs: comprehensive architecture documentation with PlantUML diagrams

- docs/ARCHITECTURE.md: full system overview, technology stack, directory
  structure, component architecture, data flow, auth, polling/caching,
  download matching pipeline, API reference, frontend architecture,
  configuration, deployment guide
- docs/diagrams/component.puml: system component diagram
- docs/diagrams/seq-auth.puml: authentication sequence diagram
- docs/diagrams/seq-dashboard.puml: dashboard request sequence diagram
- docs/diagrams/seq-polling.puml: background polling cycle sequence
- docs/diagrams/class-server.puml: server-side class/module diagram
- docs/diagrams/class-data.puml: data model / entity diagram
- docs/diagrams/state-ui.puml: frontend UI state diagram
- docs/diagrams/state-poller.puml: poller state diagram
- docs/diagrams/activity-matching.puml: download matching activity diagram

docs/diagrams/seq-auth.puml

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+@startuml seq-auth
+!theme plain
+title sofarr — Authentication Sequence
+
+actor User as user
+participant "Browser\n(app.js)" as browser
+participant "Express\n/api/auth" as auth
+participant "Emby\nServer" as emby
+
+== Page Load ==
+user -> browser : Navigate to sofarr
+activate browser
+browser -> auth : GET /api/auth/me
+activate auth
+auth -> auth : Read emby_user cookie
+alt Cookie exists and valid
+  auth --> browser : { authenticated: true, user: { name, isAdmin } }
+  browser -> browser : showDashboard()
+  browser -> browser : fetchUserDownloads(true)
+  browser -> browser : startAutoRefresh()
+  browser -> browser : dismissSplash()
+else No cookie
+  auth --> browser : { authenticated: false }
+  browser -> browser : dismissSplash()
+  browser -> browser : showLogin()
+end
+deactivate auth
+
+== Login ==
+user -> browser : Enter username + password
+browser -> auth : POST /api/auth/login\n{ username, password }
+activate auth
+auth -> emby : POST /Users/authenticatebyname\n{ Username, Pw }
+activate emby
+alt Valid credentials
+  emby --> auth : { User: { Id, ... }, AccessToken }
+  auth -> emby : GET /Users/{userId}
+  emby --> auth : { Name, Policy: { IsAdministrator } }
+  deactivate emby
+  auth -> auth : Set httpOnly cookie\nemby_user = { id, name, isAdmin, token }\n(24h TTL)
+  auth --> browser : { success: true, user: { name, isAdmin } }
+  browser -> browser : fadeOutLogin()
+  browser -> browser : showSplash()
+  browser -> browser : showDashboard()
+  browser -> browser : fetchUserDownloads(true)
+  browser -> browser : startAutoRefresh()
+  browser -> browser : dismissSplash()
+else Invalid credentials
+  emby --> auth : 401 Error
+  deactivate emby
+  auth --> browser : { success: false, error: "Invalid..." }
+  browser -> browser : showLoginError()
+end
+deactivate auth
+
+== Logout ==
+user -> browser : Click Logout
+browser -> browser : stopAutoRefresh()
+browser -> auth : POST /api/auth/logout
+activate auth
+auth -> auth : Clear emby_user cookie
+auth --> browser : { success: true }
+deactivate auth
+browser -> browser : showLogin()
+
+deactivate browser
+@enduml