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

[ntatschner]

Display on an anyhow::Error prints only the outermost context. Every mail path wraps its errors (.context("parse To address"), .context("SMTP send failed"), …), so error = %e has been logging the wrapper and silently discarding the actual cause.

That is why the waitlist invite outage read as a broken transport for as long as it did. The logs only ever said:

waitlist invite RESEND failed — the mail transport is still broken   error="parse To address"

The cause — that a display name containing @ is not a valid RFC 5322 atom — was one context layer down and never printed. The transport was fine the whole time.

Change

The 8 sites that log a Mailer error now use %format!("{e:#}"). The alternate Display form flattens the whole chain onto one line:

error="send_waitlist_invite failed: parse To address: invalid email address: a@b.com <a@b.com>"

Single-line matters. ?e (Debug) also carries the chain but emits multi-line output with Caused by: stanzas, which is hostile to the JSON log pipeline.

File	Sites
waitlist_routes.rs	3 (invite on signup, admit, resend)
auth_routes.rs	4 (verification ×2, password reset, email-change verify)
magic_link_routes.rs	1
smtp_admin_routes.rs	1 (see below)

Not a new idiom. format!("{e:#}") is already used for exactly this at smtp_admin_routes.rs:351 and telemetry.rs:63. The SMTP test-send site is included because its code used ?e while the comment directly above it prescribed {e:#} — code and comment now agree.

Deliberately not a repo-wide sweep

There are 392 error = %e sites. Most log a thiserror enum whose Display already interpolates its source — e.g. WaitlistError::Database(#[from] sqlx::Error) renders as "database error: {0}" — so {:#} would add nothing there. Only anyhow errors lose information this way, and the Mailer trait (anyhow::Result) is where it actually bit. Each of the 8 edits was verified by its surrounding context to be a mailer call site, not just by line number.

Test

alternate_display_surfaces_the_cause_chain_that_plain_display_hides pins the difference directly: plain Display yields only "send_waitlist_invite failed" and demonstrably hides the root cause, while {e:#} carries every layer down to it — and stays single-line.

cargo test -p starstats-server 960 + 932, clippy -D warnings and cargo fmt --all --check clean. No migration, no behaviour change beyond log content.