feat!: remove direct messaging (chat)

E2EE direct messages, the chat dock, the Messages module, the message
WebSocket and its realtime registry, and the messages / message_keys
tables (migration 0097; the downgrade restores the empty schema). Prod
held two welcome notes, no message between people and one key — never
used (owner decision, E34). WebSocket now has no owner module: the lint
bans it everywhere. Privacy policy updated: it no longer promises E2EE
for a feature that is gone.

frontend/src/components/PlexPlayer.tsx

@@ -1348,7 +1348,7 @@ export default function PlexPlayer({ itemId, onClose, queue, fromStart = false }
   const currentAudio = detail?.audio_streams.find((x) => x.ord === (audioOrd ?? 0));
   const currentSub = detail?.subtitle_streams.find((x) => x.ord === subOrd);
 
-  // Portaled to <body>, like every other full-screen overlay here (Modal, PlayerModal, ChatDock,
+  // Portaled to <body>, like every other full-screen overlay here (Modal, PlayerModal,
   // BackToTop). A fixed, full-viewport player must not be painted inside the page scroller: the
   // moment that scroller gets a mask/filter/transform/contain it becomes a stacking context, and
   // this z-overlay is then scoped inside it — the rail, the side panel and the header paint straight