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-polling.puml

@@ -0,0 +1,89 @@
+@startuml seq-polling
+!theme plain
+title sofarr — Background Polling Cycle
+
+participant "index.js\n(startup)" as entry
+participant "Poller" as poller
+participant "Config" as config
+participant "SABnzbd\n(per instance)" as sab
+participant "Sonarr\n(per instance)" as sonarr
+participant "Radarr\n(per instance)" as radarr
+participant "qBittorrent\nClient" as qbt
+participant "MemoryCache" as cache
+
+== Startup ==
+entry -> poller : startPoller()
+activate poller
+
+alt POLL_INTERVAL > 0
+  poller -> poller : pollAllServices() (immediate)
+  poller -> poller : setInterval(pollAllServices,\nPOLL_INTERVAL)
+else POLL_INTERVAL = 0
+  poller --> entry : "Polling disabled, on-demand mode"
+end
+
+== Poll Cycle ==
+poller -> poller : Check: polling flag?\n(skip if concurrent)
+poller -> poller : polling = true
+poller -> poller : start = Date.now()
+
+poller -> config : getSABnzbdInstances()
+config --> poller : [{ id, url, apiKey }]
+poller -> config : getSonarrInstances()
+config --> poller : [{ id, url, apiKey }]
+poller -> config : getRadarrInstances()
+config --> poller : [{ id, url, apiKey }]
+
+note over poller : All fetches run in\nparallel via Promise.all,\neach wrapped in timed()
+
+par SABnzbd Queue
+  poller -> sab : GET /api?mode=queue
+  sab --> poller : { queue: { slots, status, speed } }
+and SABnzbd History
+  poller -> sab : GET /api?mode=history&limit=10
+  sab --> poller : { history: { slots } }
+and Sonarr Tags
+  poller -> sonarr : GET /api/v3/tag
+  sonarr --> poller : [{ id, label }]
+and Sonarr Queue
+  poller -> sonarr : GET /api/v3/queue\n?includeSeries=true
+  sonarr --> poller : { records: [{ seriesId, series, ... }] }
+and Sonarr History
+  poller -> sonarr : GET /api/v3/history\n?pageSize=10
+  sonarr --> poller : { records: [{ seriesId, ... }] }
+and Radarr Queue
+  poller -> radarr : GET /api/v3/queue\n?includeMovie=true
+  radarr --> poller : { records: [{ movieId, movie, ... }] }
+and Radarr History
+  poller -> radarr : GET /api/v3/history\n?pageSize=10
+  radarr --> poller : { records: [{ movieId, ... }] }
+and Radarr Tags
+  poller -> radarr : GET /api/v3/tag
+  radarr --> poller : [{ id, label }]
+and qBittorrent
+  poller -> qbt : getTorrents()
+  qbt --> poller : [{ name, progress, ... }]
+end
+
+poller -> poller : Record per-task timings\nlastPollTimings = { totalMs,\ntimestamp, tasks: [{label, ms}] }
+
+poller -> poller : cacheTTL = POLL_INTERVAL × 3
+
+poller -> cache : set('poll:sab-queue', ..., cacheTTL)
+poller -> cache : set('poll:sab-history', ..., cacheTTL)
+poller -> cache : set('poll:sonarr-tags', ..., cacheTTL)
+
+note over poller : Tag queue records with\n_instanceUrl on embedded\nseries/movie objects
+
+poller -> cache : set('poll:sonarr-queue', ..., cacheTTL)
+poller -> cache : set('poll:sonarr-history', ..., cacheTTL)
+poller -> cache : set('poll:radarr-queue', ..., cacheTTL)
+poller -> cache : set('poll:radarr-history', ..., cacheTTL)
+poller -> cache : set('poll:radarr-tags', ..., cacheTTL)
+poller -> cache : set('poll:qbittorrent', ..., cacheTTL)
+
+poller -> poller : polling = false\nlog elapsed time
+
+deactivate poller
+
+@enduml