feat: add PATCH /federation/me/also-known-as endpoint

Adds alsoKnownAs column to users table (migration 013), reads it in
the AP actor JSON, and exposes PATCH /federation/me/also-known-as to
set or clear it. Required pre-condition for broadcast_move.

crates/adapters/postgres/migrations/013_also_known_as.sql

@@ -0,0 +1 @@
+ALTER TABLE users ADD COLUMN IF NOT EXISTS also_known_as TEXT;