fix(server): log the full anyhow cause chain at mailer failure sites

crates/starstats-server/src/auth_routes.rs

@@ -370,7 +370,7 @@ pub async fn signup<U: UserStore>(
         .send_verification(&user.email, &user.claimed_handle, &token)
         .await
     {
-        tracing::warn!(error = %e, user_id = %user.id, "send verification email failed");
+        tracing::warn!(error = %format!("{e:#}"), user_id = %user.id, "send verification email failed");
     }
 
     issue_token(&issuer, &user.id.to_string(), &user.claimed_handle)
@@ -580,7 +580,7 @@ pub async fn resend_verification<U: UserStore>(
         .send_verification(&user.email, &user.claimed_handle, &token)
         .await
     {
-        tracing::warn!(error = %e, user_id = %user.id, "send verification email failed");
+        tracing::warn!(error = %format!("{e:#}"), user_id = %user.id, "send verification email failed");
     }
 
     if let Err(e) = audit
@@ -947,7 +947,7 @@ pub async fn password_reset_start<U: UserStore>(
             .send_password_reset(&user.email, &user.claimed_handle, &token)
             .await
         {
-            tracing::warn!(error = %e, user_id = %user.id, "send reset email failed");
+            tracing::warn!(error = %format!("{e:#}"), user_id = %user.id, "send reset email failed");
         }
     } else {
         // Sleep briefly to flatten the timing oracle — a fast
@@ -1127,7 +1127,7 @@ pub async fn email_change_start<U: UserStore>(
         .send_email_change_verify(&new_email, &auth.preferred_username, &token)
         .await
     {
-        tracing::warn!(error = %e, "send email-change verify failed");
+        tracing::warn!(error = %format!("{e:#}"), "send email-change verify failed");
     }
     (
         StatusCode::OK,

crates/starstats-server/src/magic_link_routes.rs

@@ -139,7 +139,7 @@ pub async fn start<U: UserStore, M: MagicLinkStore>(
         .send_magic_link(&user.email, &user.claimed_handle, &token)
         .await
     {
-        tracing::warn!(error = %e, "send magic link failed (best-effort)");
+        tracing::warn!(error = %format!("{e:#}"), "send magic link failed (best-effort)");
     }
 
     Json(MagicLinkStartResponse { sent: true }).into_response()

crates/starstats-server/src/mail.rs

@@ -703,6 +703,48 @@ pub mod test_support {
 mod tests {
     use super::*;
 
+    /// Why the mailer log sites format with `{e:#}` and not `%e`.
+    ///
+    /// `Display` on an `anyhow::Error` prints ONLY the outermost context.
+    /// Every mail failure is wrapped (`.context("parse To address")`,
+    /// `.context("SMTP send failed")`, …), so `error = %e` logs the wrapper
+    /// and silently discards the actual cause. That is exactly how the
+    /// waitlist invite outage read as "the mail transport is broken" for as
+    /// long as it did — the logs only ever said `parse To address`, never
+    /// which address or why.
+    ///
+    /// The alternate form `{e:#}` flattens the whole chain onto one line,
+    /// which keeps it greppable in the JSON log pipeline (unlike `?e`,
+    /// whose `Debug` output is multi-line).
+    #[test]
+    fn alternate_display_surfaces_the_cause_chain_that_plain_display_hides() {
+        let err = anyhow::anyhow!("invalid email address: a@b.com <a@b.com>")
+            .context("parse To address")
+            .context("send_waitlist_invite failed");
+
+        let plain = format!("{err}");
+        let chained = format!("{err:#}");
+
+        // What `%e` would have logged: the wrapper only.
+        assert_eq!(plain, "send_waitlist_invite failed");
+        assert!(
+            !plain.contains("invalid email address"),
+            "plain Display hides the root cause — this is the bug being guarded against"
+        );
+
+        // What `{e:#}` logs: every layer, root cause included.
+        assert!(chained.contains("send_waitlist_invite failed"));
+        assert!(chained.contains("parse To address"));
+        assert!(
+            chained.contains("invalid email address: a@b.com <a@b.com>"),
+            "the root cause must survive into the log line, got {chained}"
+        );
+        assert!(
+            !chained.contains('\n'),
+            "must stay single-line for the JSON log pipeline, got {chained}"
+        );
+    }
+
     /// Repro for the production failure: every waitlist invite/resend
     /// died with `parse To address`.
     ///

crates/starstats-server/src/smtp_admin_routes.rs

@@ -344,7 +344,7 @@ pub async fn test_smtp<C: SmtpConfigStore, U: UserStore>(
         // without it the user only sees the outermost wrap ("SMTP send
         // failed") and the actual cause (auth rejected, TLS handshake,
         // connection refused, …) is invisible.
-        tracing::warn!(error = ?e, "smtp test send failed");
+        tracing::warn!(error = %format!("{e:#}"), "smtp test send failed");
         return error(
             StatusCode::BAD_GATEWAY,
             "smtp_send_failed",

crates/starstats-server/src/waitlist_routes.rs

@@ -198,7 +198,7 @@ pub async fn join(
             // and the row looks perfectly admitted from the DB side.
             if let Err(e) = mailer.send_waitlist_invite(&req.email, invite_token).await {
                 tracing::error!(
-                    error = %e,
+                    error = %format!("{e:#}"),
                     email = %req.email,
                     "waitlist invite send FAILED — admitted user has no link; re-admit from /admin/waitlist"
                 );
@@ -331,7 +331,7 @@ pub async fn admin_admit(
             .await
         {
             tracing::error!(
-                error = %e,
+                error = %format!("{e:#}"),
                 email = %inv.email,
                 "waitlist invite send FAILED for an admitted user — they have no link"
             );
@@ -387,7 +387,7 @@ pub async fn admin_resend(
             .await
         {
             tracing::error!(
-                error = %e,
+                error = %format!("{e:#}"),
                 email = %inv.email,
                 "waitlist invite RESEND failed — the mail transport is still broken"
             );