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/state-poller.puml

@@ -0,0 +1,65 @@
+@startuml state-poller
+!theme plain
+title sofarr — Poller State Diagram
+
+[*] --> CheckConfig : startPoller()
+
+state CheckConfig <<choice>>
+CheckConfig --> Disabled : POLL_INTERVAL = 0\nor 'off' / 'false'
+CheckConfig --> Idle : POLL_INTERVAL > 0
+
+state Disabled {
+  state "On-demand mode\nNo background timer" as od
+  od : Data fetched only when\na dashboard request\nfinds empty cache
+}
+
+Disabled --> Polling : pollAllServices()\n(triggered by dashboard request)
+Polling --> Disabled : Poll complete\n(return to on-demand)
+
+state Idle {
+  state "Waiting for\nnext interval" as waiting
+}
+
+Idle --> Polling : setInterval fires\nor immediate first poll
+
+state Polling {
+  state "polling = true" as lock
+  state "Fetching all services\n(Promise.all)" as fetching
+  state "Storing results\nin cache" as storing
+  state "Recording timings" as timing
+
+  [*] --> lock
+  lock --> fetching
+  fetching --> storing : All promises resolved
+  fetching --> ErrorState : Any individual service\nerror (caught per-service)
+  storing --> timing
+  timing --> [*] : polling = false
+}
+
+state ErrorState as "Handle Error" {
+  state "Log error\npolling = false" as err
+}
+
+ErrorState --> Idle : Next interval
+Polling --> Idle : Poll complete\n(back to waiting)
+
+state "Concurrent Poll\nAttempt" as skip {
+  state "polling === true\n→ skip" as sk
+}
+
+Idle --> skip : Interval fires while\nprevious still running
+skip --> Idle : Log "still running,\nskipping"
+
+note right of Polling
+  **Cache TTL**: POLL_INTERVAL × 3
+  Ensures data survives between polls
+  even if one cycle is slow.
+end note
+
+note right of Disabled
+  **Cache TTL**: 30000ms (30s)
+  After expiry, next dashboard
+  request triggers a fresh poll.
+end note
+
+@enduml