feat(auth): support optional expiry for bot API tokens

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/domain/api_token.rs

@@ -12,4 +12,6 @@ pub struct ApiToken {
     pub token_hash: Vec<u8>,
     pub created_at: DateTime<Utc>,
     pub last_used_at: Option<DateTime<Utc>>,
+    /// When the token stops authenticating. `None` = never expires.
+    pub expires_at: Option<DateTime<Utc>>,
 }