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/ui/NavRail.tsx

@@ -112,7 +112,7 @@ export function NavRailItem({
         }}
         // With the label visible the row's name starts with that text (WCAG 2.5.3: the spoken name
         // contains the visible one); the full name stays in the tooltip. Icon-only, the full name is the
-        // name. The exact count rides in the name either way, so "Messages (3)" is heard and not only
+        // name. The exact count rides in the name either way, so "Alerts (3)" is heard and not only
         // seen; the drawn badge is hidden (labelled, it was read first and capped: "9+Alerts").
         aria-label={`${showLabel ? shortLabel : label}${shown ? ` (${badge})` : ""}`}
         aria-current={active ? "page" : undefined}