#21: fill in message metadata and stop losing mail to a missing folder

Two problems the mail screens exposed, both in the delivery path.

The subject, snippet, sender name and message-id columns have existed
since the first migration, but nothing ever wrote them: every row went in
with those fields NULL. A message list built on the database would have
shown blank subjects for every message. Delivery now parses them once per
message and stores them, decoding RFC 2047 encoded words so an accented
subject is readable rather than raw. Parsing failures are non-fatal: a
message with unreadable headers is still delivered, just with less
metadata, because refusing it would lose mail over a malformed Subject.

Worse, a message the spam filter routed to Junk failed delivery outright
when the mailbox had no Junk folder - the sender got 451 and retried
forever against a mailbox that would never grow one on its own. Delivery
now falls back to INBOX and says so in the log. Found by sending a real
message through port 25 against the running stack.

The SMTP handler also swallowed the cause of any unexpected failure: the
peer saw "temporary error" and the operator got nothing. It is logged now,
which is how the missing folder was diagnosed at all.

Also adds the JWT verifier the mail API needs. The tokens are minted by
the auth service and verified here under the same secret, so the test
fixtures are real tokens produced by that service: if the two
implementations drift, this fails before the webmail does. Challenge
tokens, wrong secrets, tampered payloads, expired tokens and the other
surface's audience are all rejected.

The dev seed gained the standard folder set, which is what a provisioned
mailbox has and what the fallback above should not have to compensate for.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

services/protocols/internal/presentation/smtp/backend.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"errors"
 	"io"
+	"log"
 	"net"
 
 	gosmtp "github.com/emersion/go-smtp"
@@ -157,6 +158,10 @@ func (s *session) Data(r io.Reader) error {
 		// Anything else is an internal failure. It is reported as temporary so
 		// the sender retries: the message was not stored, and telling the peer
 		// it failed permanently would discard mail over a bug on this side.
+		//
+		// Logged because the peer only ever sees "temporary error"; without
+		// this line an operator has a bounced delivery and no cause.
+		log.Printf("smtp: could not store message from %s: %v", s.from, err)
 		return &gosmtp.SMTPError{
 			Code:         451,
 			EnhancedCode: gosmtp.EnhancedCode{4, 3, 0},