fix(PROJ-57): UTF-8-Encoding für Mails mit Nicht-UTF-8-Charset korrigieren

Der Mail-Parser ignorierte das charset-Parameter aus Content-Type und
interpretierte Bytes immer als UTF-8, wodurch iso-8859-1/windows-1252
kodierte Mails (z.B. mit Umlauten) als Mojibake gespeichert wurden.
Zusätzlich fehlte das Charset für die Manticore-MySQL-Verbindung und
der charset-Parameter im JSON-Response-Header.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pkg/mailparser/parser.go

@@ -11,6 +11,8 @@ import (
 	"net/mail"
 	"strings"
 	"time"
+
+	"golang.org/x/text/encoding/htmlindex"
 )
 
 // Attachment represents a MIME attachment in a parsed email.
@@ -184,6 +186,7 @@ func Parse(raw []byte) (*ParsedMail, error) {
 	} else {
 		body, _ := io.ReadAll(msg.Body)
 		decoded := decodeBody(body, msg.Header.Get("Content-Transfer-Encoding"))
+		decoded = decodeCharset(decoded, params["charset"])
 		if strings.Contains(mediaType, "html") {
 			pm.HTMLBody = string(decoded)
 		} else {
@@ -216,6 +219,9 @@ func parseMultipart(pm *ParsedMail, body io.Reader, boundary string) error {
 		data, _ := io.ReadAll(part)
 		cte := part.Header.Get("Content-Transfer-Encoding")
 		decoded := decodeBody(data, cte)
+		if strings.Contains(mediaType, "text/") {
+			decoded = decodeCharset(decoded, params["charset"])
+		}
 
 		// Check disposition for attachment
 		disp := part.Header.Get("Content-Disposition")
@@ -254,6 +260,24 @@ func parseMultipart(pm *ParsedMail, body io.Reader, boundary string) error {
 	return nil
 }
 
+// decodeCharset converts data from the declared MIME charset to UTF-8.
+// Empty charset or "utf-8"/"us-ascii" are passed through unchanged.
+func decodeCharset(data []byte, charset string) []byte {
+	charset = strings.ToLower(strings.TrimSpace(charset))
+	if charset == "" || charset == "utf-8" || charset == "us-ascii" || charset == "ascii" {
+		return data
+	}
+	enc, err := htmlindex.Get(charset)
+	if err != nil {
+		return data
+	}
+	decoded, err := enc.NewDecoder().Bytes(data)
+	if err != nil {
+		return data
+	}
+	return decoded
+}
+
 // decodeBody decodes Content-Transfer-Encoding if needed.
 func decodeBody(data []byte, cte string) []byte {
 	switch strings.ToLower(strings.TrimSpace(cte)) {