fix(PROJ-86): Date-Fallback auf Received-Header-Kette wenn Date-Header fehlt

Alte Bulk-importierte Archive (kein Date:-Header, nur Received:-Zeilen)
bekamen bislang das Import-Datum (received_at) als date_ts zugewiesen -
storage.Save() ignoriert den pm.Date-Parameter komplett und setzt
received_at immer auf NOW(). Jetzt wird bei fehlendem/unparsbarem
Date:-Header die Received:-Kette geparst (origin-nächster Hop zuerst,
via net/mail.Header["Received"] rückwärts iteriert) - liefert das
tatsächliche Mail-Datum statt des Import-Zeitpunkts.

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

pkg/mailparser/parser.go

@@ -51,6 +51,69 @@ type ParsedMail struct {
 // fatal error for the whole process.
 const maxMultipartDepth = 20
 
+// parseDateFlexible tries the RFC 2822 date format plus the non-standard
+// variants seen in real-world Date: and Received: header tails (missing
+// leading zero, colon in timezone offset, no seconds, MTA timezone name,
+// localised weekday prefix, trailing "(TZ)" comment). Returns ok=false if
+// none of them match.
+func parseDateFlexible(raw string) (time.Time, bool) {
+	raw = strings.TrimSpace(raw)
+	if raw == "" {
+		return time.Time{}, false
+	}
+	// Strip parenthesised timezone comment: "... +0100 (CET)" → "... +0100"
+	if idx := strings.LastIndex(raw, "("); idx > 0 {
+		raw = strings.TrimSpace(raw[:idx])
+	}
+	for _, layout := range []string{
+		"Mon, 2 Jan 2006 15:04:05 -0700",
+		"Mon, 02 Jan 2006 15:04:05 -0700",
+		"2 Jan 2006 15:04:05 -0700",
+		"02 Jan 2006 15:04:05 -0700",
+		"Mon, 2 Jan 2006 15:04:05 MST",
+		"Mon, 02 Jan 2006 15:04:05 MST",
+		// Colon in timezone offset (e.g. "+02:00") used by some MTA versions
+		"Mon, 2 Jan 2006 15:04:05 -07:00",
+		"Mon, 02 Jan 2006 15:04:05 -07:00",
+		"2 Jan 2006 15:04:05 -07:00",
+		"02 Jan 2006 15:04:05 -07:00",
+		// Without seconds
+		"Mon, 2 Jan 2006 15:04 -0700",
+		"Mon, 02 Jan 2006 15:04 -0700",
+		"2 Jan 2006 15:04 -0700",
+		// Go stdlib aliases
+		time.RFC1123Z,
+		time.RFC1123,
+	} {
+		if t, err := time.Parse(layout, raw); err == nil {
+			return t, true
+		}
+	}
+	// Some MTAs (e.g. PMG with German locale) use localised weekday names:
+	// "So, 24 Aug 2025 00:05:17 +0200" instead of "Sun, 24 Aug 2025...".
+	// Strip the "Weekday, " prefix (≤3 chars before the first comma) and retry.
+	commaIdx := strings.Index(raw, ",")
+	if commaIdx <= 0 || commaIdx > 3 {
+		return time.Time{}, false
+	}
+	noWeekday := strings.TrimSpace(raw[commaIdx+1:])
+	for _, layout := range []string{
+		"2 Jan 2006 15:04:05 -0700",
+		"02 Jan 2006 15:04:05 -0700",
+		"2 Jan 2006 15:04:05 -07:00",
+		"02 Jan 2006 15:04:05 -07:00",
+		"2 Jan 2006 15:04:05 MST",
+		"02 Jan 2006 15:04:05 MST",
+		"2 Jan 2006 15:04 -0700",
+		"02 Jan 2006 15:04 -0700",
+	} {
+		if t, err := time.Parse(layout, noWeekday); err == nil {
+			return t, true
+		}
+	}
+	return time.Time{}, false
+}
+
 // Parse parses a raw RFC 2822 / MIME email and returns a ParsedMail.
 //
 // Input is untrusted (IMAP/POP3/SMTP/upload). A panic inside the MIME/charset
@@ -123,72 +186,35 @@ func Parse(raw []byte) (pmOut *ParsedMail, errOut error) {
 		}
 	}
 
-	// Date — try go-message parser first, then fallback formats, then zero
+	// Date — try go-message parser first, then fallback formats, then Received:
+	// trace headers, then zero.
 	if d, err := msg.Header.Date(); err == nil {
 		pm.Date = d
+	} else if t, ok := parseDateFlexible(msg.Header.Get("Date")); ok {
+		pm.Date = t
 	} else {
-		// Some MUAs emit non-standard variants (e.g. "+0100 (CET)" suffix).
+		// No usable Date: header at all (missing, or unparseable in every
-		// Try common RFC 2822 / non-standard formats before giving up.
+		// format we know) — some very old/legacy MTAs never set one. Fall
-		raw := strings.TrimSpace(msg.Header.Get("Date"))
+		// back to the Received: trace, which every relay hop adds a
-		// Strip parenthesised timezone comment: "... +0100 (CET)" → "... +0100"
+		// timestamp to. net/mail.Header["Received"] preserves header order
-		if idx := strings.LastIndex(raw, "("); idx > 0 {
+		// (topmost/latest hop first, origin-closest hop last) — iterate
-			raw = strings.TrimSpace(raw[:idx])
+		// back-to-front so we try the origin-closest hop first, which is
-		}
+		// our best proxy for when the mail actually originated. Far better
-		parsed := false
+		// than defaulting to "now"/import time for archived mail.
-		for _, layout := range []string{
-			"Mon, 2 Jan 2006 15:04:05 -0700",
-			"Mon, 02 Jan 2006 15:04:05 -0700",
-			"2 Jan 2006 15:04:05 -0700",
-			"02 Jan 2006 15:04:05 -0700",
-			"Mon, 2 Jan 2006 15:04:05 MST",
-			"Mon, 02 Jan 2006 15:04:05 MST",
-			// Colon in timezone offset (e.g. "+02:00") used by some MTA versions
-			"Mon, 2 Jan 2006 15:04:05 -07:00",
-			"Mon, 02 Jan 2006 15:04:05 -07:00",
-			"2 Jan 2006 15:04:05 -07:00",
-			"02 Jan 2006 15:04:05 -07:00",
-			// Without seconds
-			"Mon, 2 Jan 2006 15:04 -0700",
-			"Mon, 02 Jan 2006 15:04 -0700",
-			"2 Jan 2006 15:04 -0700",
-			// Go stdlib aliases
-			time.RFC1123Z,
-			time.RFC1123,
-		} {
-			if t, err := time.Parse(layout, raw); err == nil {
-				pm.Date = t
-				parsed = true
-				break
-			}
-		}
-		if !parsed {
-			// Some MTAs (e.g. PMG with German locale) use localised weekday names:
-			// "So, 24 Aug 2025 00:05:17 +0200" instead of "Sun, 24 Aug 2025...".
-			// Strip the "Weekday, " prefix (≤3 chars before the first comma) and retry.
-			if commaIdx := strings.Index(raw, ","); commaIdx > 0 && commaIdx <= 3 {
-				noWeekday := strings.TrimSpace(raw[commaIdx+1:])
-				for _, layout := range []string{
-					"2 Jan 2006 15:04:05 -0700",
-					"02 Jan 2006 15:04:05 -0700",
-					"2 Jan 2006 15:04:05 -07:00",
-					"02 Jan 2006 15:04:05 -07:00",
-					"2 Jan 2006 15:04:05 MST",
-					"02 Jan 2006 15:04:05 MST",
-					"2 Jan 2006 15:04 -0700",
-					"02 Jan 2006 15:04 -0700",
-				} {
-					if t, err := time.Parse(layout, noWeekday); err == nil {
-						pm.Date = t
-						parsed = true
-						break
-					}
-				}
-			}
-		}
-		if !parsed {
-			// Leave pm.Date as zero — storage will use DB DEFAULT NOW()
 		pm.Date = time.Time{}
+		received := msg.Header["Received"]
+		for i := len(received) - 1; i >= 0; i-- {
+			semi := strings.LastIndex(received[i], ";")
+			if semi < 0 {
+				continue
 			}
+			if t, ok := parseDateFlexible(received[i][semi+1:]); ok {
+				pm.Date = t
+				break
+			}
+		}
+		// Leave pm.Date as zero if nothing parsed — storage will use DB
+		// DEFAULT NOW() / EffectiveDate's caller-supplied fallback.
 	}
 
 	// Parse body / MIME parts