Add DM support, friend identification, and target pruning

React in 1:1 DMs alongside servers/groups, surface the resolved friend
username at startup and in the stats panel, verify reactions actually
stick after adding them (with jump links for manual spot-checking),
and prune stale targets from stats on each run so only the currently
mutual set is shown.

config.example.json

@@ -5,5 +5,6 @@
   "days_back": 10,
   "db_path": "reactions.sqlite3",
   "log_level": "INFO",
-  "manual_guild_ids": []
+  "manual_guild_ids": [],
+  "verify_reactions": true
 }