feat: direct messages

1:1 conversations (userAId < userBId dedup pair), messages,
per-participant read tracking (conversation_reads). Block relationship
is enforced on every send. UI: /messages list + /messages/[id] thread
(5s poll), MessageButton on profiles, unread-badged nav icon.

This completes the social-feature backlog: notifications, rate
limiting, blocking, reporting, search/discovery, mentions, DMs, plus
fixes for the recipe-visibility 404, follow race, and 2-level comment
thread cap found during the earlier audit.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

packages/db/src/migrations/meta/_journal.json

@@ -141,6 +141,13 @@
       "when": 1783108786252,
       "tag": "0019_clumsy_leader",
       "breakpoints": true
+    },
+    {
+      "idx": 20,
+      "version": "7",
+      "when": 1783109825851,
+      "tag": "0020_acoustic_exiles",
+      "breakpoints": true
     }
   ]
 }