feat: user tombstone and retired usernames

crates/adapters/sqlite/migrations/0102_account_deletion.sql

@@ -0,0 +1,6 @@
+ALTER TABLE users ADD COLUMN deleted_at TEXT;
+
+CREATE TABLE retired_usernames (
+    username TEXT PRIMARY KEY COLLATE NOCASE,
+    retired_at TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now'))
+);