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/component.puml

@@ -0,0 +1,94 @@
+@startuml component
+!theme plain
+title sofarr — Component Diagram
+
+skinparam componentStyle rectangle
+skinparam packageStyle frame
+
+package "Browser" as browser {
+  [index.html] as html
+  [app.js] as appjs
+  [style.css] as css
+  html ..> appjs : loads
+  html ..> css : loads
+}
+
+package "Express Server" as server {
+
+  package "Middleware" {
+    [CORS] as cors
+    [cookie-parser] as cp
+    [express.json] as ej
+    [express.static] as es
+  }
+
+  package "Routes" as routes {
+    [auth.js\n/api/auth] as auth
+    [dashboard.js\n/api/dashboard] as dashboard
+    [emby.js\n/api/emby] as emby_route
+    [sabnzbd.js\n/api/sabnzbd] as sab_route
+    [sonarr.js\n/api/sonarr] as sonarr_route
+    [radarr.js\n/api/radarr] as radarr_route
+  }
+
+  package "Utilities" as utils {
+    [poller.js] as poller
+    [cache.js\nMemoryCache] as cache
+    [config.js] as config
+    [qbittorrent.js\nQBittorrentClient] as qbt
+    [logger.js] as logger
+  }
+
+  [index.js\nEntry Point] as entry
+
+  entry --> cors
+  entry --> cp
+  entry --> ej
+  entry --> es
+  entry --> auth
+  entry --> dashboard
+  entry --> emby_route
+  entry --> sab_route
+  entry --> sonarr_route
+  entry --> radarr_route
+  entry --> poller : startPoller()
+
+  dashboard --> cache : read poll:* keys
+  dashboard --> poller : pollAllServices()\n(on-demand mode)
+  dashboard --> config : getSonarrInstances()\ngetRadarrInstances()
+  dashboard --> qbt : mapTorrentToDownload()
+
+  poller --> cache : set poll:* keys
+  poller --> config : get all instances
+  poller --> qbt : getTorrents()
+  poller --> logger
+
+  qbt --> config : getQbittorrentInstances()
+  qbt --> logger
+}
+
+cloud "External Services" as external {
+  [Emby / Jellyfin] as emby
+  [SABnzbd] as sab
+  [Sonarr] as sonarr
+  [Radarr] as radarr
+  [qBittorrent] as qbit
+}
+
+auth --> emby : authenticate\nuser profile
+dashboard ..> emby : /user-summary\n(live fetch)
+emby_route --> emby
+sab_route --> sab
+sonarr_route --> sonarr
+radarr_route --> radarr
+
+poller --> sab : queue + history
+poller --> sonarr : tags + queue + history
+poller --> radarr : tags + queue + history
+qbt --> qbit : login + torrents/info
+
+appjs --> auth : POST /login\nGET /me
+appjs --> dashboard : GET /user-downloads\nGET /status
+es --> html : serve static
+
+@enduml