fix(PROJ-86): Manticore-Index-Drift (emails_global) + Datumssortierung bei fehlendem Date-Header

emails_global bekam nie Tenant-Mails gespiegelt, Superadmin-Suche sah nur
~30% aller Mails. Zusätzlich sanken Mails ohne gültigen Date-Header
(date_ts=0) beim datumssortierten Listing ans Ende aller Ergebnisse und
waren dadurch praktisch unauffindbar ("GUI zeigt neue Mails nicht") -
Import/Indexierung liefen technisch korrekt, nur Sortierung/Spiegelung
waren kaputt.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UPFC6Jk2ke1Pq9XcuVGP1R

internal/index/index.go

@@ -5,6 +5,18 @@ import (
 	"time"
 )
 
+// EffectiveDate returns headerDate for sorting/filtering unless it is the
+// zero value (missing or unparseable Date: header — common in spam), in
+// which case fallback (typically received_at) is used instead. Without this,
+// mails with a bad Date header get date_ts=0 and sink to the bottom of every
+// date-sorted search result, effectively hiding them from the mail list.
+func EffectiveDate(headerDate, fallback time.Time) time.Time {
+	if headerDate.IsZero() {
+		return fallback
+	}
+	return headerDate
+}
+
 // MailDocument is the indexed representation of a stored email.
 type MailDocument struct {
 	ID             string