feat(ai-moderation): add user profile self-learning system

Add user_profiles table, store, and background learner worker
that summarizes user communication style, topics, and personality.

- New user_profiles table (user_id PK, guild_id, profile_summary, last_analyzed_at)
- userProfileStore.ts — CRUD (get/update) following channelCultureStore pattern
- userProfileLearner.ts — background worker: queries 100 recent msgs per user,
  calls LLM for personality summary, updates every 12h
- Inject <user_profile> XML tag per-message in moderation prompt
- Start worker alongside cultureLearner in aiAnalyzer.ts
- Migration 0008 for user_profiles table

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

services/discord-gateway/drizzle/migrations/meta/_journal.json

@@ -57,6 +57,13 @@
       "when": 1781174400000,
       "tag": "0007_mascot_chat_table",
       "breakpoints": true
+    },
+    {
+      "idx": 8,
+      "version": "7",
+      "when": 1781270000000,
+      "tag": "0008_add_user_profiles_table",
+      "breakpoints": true
     }
   ]
 }