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;

crates/adapters/postgres/src/user/mod.rs

@@ -269,12 +269,12 @@ impl UserWriter for PgUserRepository {
     ) -> Result<(), DomainError> {
         sqlx::query(
             "UPDATE users SET \
-             display_name = COALESCE($2, display_name), \
-             bio          = COALESCE($3, bio), \
-             avatar_url   = COALESCE($4, avatar_url), \
-             header_url   = COALESCE($5, header_url), \
-             custom_css   = COALESCE($6, custom_css), \
-             updated_at   = NOW() \
+             display_name  = COALESCE($2, display_name), \
+             bio           = COALESCE($3, bio), \
+             avatar_url    = COALESCE($4, avatar_url), \
+             header_url    = COALESCE($5, header_url), \
+             custom_css    = COALESCE($6, custom_css), \
+             updated_at    = NOW() \
              WHERE id = $1",
         )
         .bind(user_id.as_uuid())
@@ -288,6 +288,20 @@ impl UserWriter for PgUserRepository {
         .into_domain()
         .map(|_| ())
     }
+
+    async fn set_also_known_as(
+        &self,
+        user_id: &UserId,
+        value: Option<String>,
+    ) -> Result<(), DomainError> {
+        sqlx::query("UPDATE users SET also_known_as = $2, updated_at = NOW() WHERE id = $1")
+            .bind(user_id.as_uuid())
+            .bind(value)
+            .execute(&self.pool)
+            .await
+            .into_domain()
+            .map(|_| ())
+    }
 }
 
 #[cfg(test)]