The /img/<server>/<message> endpoint accepted a src=<url> query
parameter and proxied the body of that URL back to the caller. Nothing
in the codebase ever produces a src= parameter — the parser only
inserts a plain tracking pixel and rewrites href links — so this branch
is dead code inherited from the original AppMail import.
Drop the src branch: requests with src now return 400. The no-src path
that serves the tracking pixel and records loads is unchanged, and a
spec covers both the pixel-serving path and the removed branch.
