Render every outgoing e-mail from an editable text template

The wording of the dunning and SEPA pre-notification mails was hard-coded in
Python, so adjusting a single sentence required a new release. Both texts now
live in plain text templates that ship as defaults, are copied into the store's
templates/mail/ directory on first start and can be edited there or under
Optionen -> E-Mail-Vorlagen; an existing file is never overwritten and a deleted
one is restored from the shipped default.

A template carries its subject in the first line and the body after a blank
line. Placeholders use the same {{ ... }} syntax as the document templates and
share their member/organization values, so a name means the same thing in a
letter and in the mail that carries it. Unknown placeholders are rejected while
editing instead of during a send run, a line holding nothing but placeholders
that render empty is dropped, and {{#claims}} ... {{/claims}} repeats per entry.

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

src/ccma/services/sepa_mail.py

@@ -5,11 +5,10 @@ import re
 from contextlib import nullcontext
 from dataclasses import dataclass
 from datetime import date, datetime
-from email.message import EmailMessage
-from email.policy import SMTP
-from email.utils import format_datetime, make_msgid
 from pathlib import Path
 
+from ccma.domain.models import Member
+from ccma.services.mail_composition import compose_mail, render_template, template_values
 from ccma.services.mail_delivery import (
     append_message,
     ensure_imap_folder,
@@ -47,55 +46,35 @@ def _available_path(directory: Path, filename: str) -> Path:
 
 
 def debit_mail_bytes(
+    repository: MemberRepository,
     *,
-    recipient: str,
-    first_name: str,
+    member: Member,
     debit: DirectDebit,
     collection_date: date,
-    creditor_id: str,
+    organization: dict,
     sender_name: str,
     sender_email: str,
     signature: str,
     created_at: datetime | None = None,
 ) -> bytes:
-    if not recipient.strip():
-        raise RepositoryError("Für das Mitglied ist keine E-Mail-Adresse hinterlegt.")
-    if not sender_email.strip() or "@" not in sender_email:
-        raise RepositoryError("Für die Info-Mails ist eine gültige Absenderadresse erforderlich.")
-    timestamp = created_at or datetime.now().astimezone()
-    message = EmailMessage(policy=SMTP)
-    message["Message-ID"] = make_msgid(domain=sender_email.rsplit("@", 1)[-1])
-    message["Date"] = format_datetime(timestamp)
-    message["From"] = f"{sender_name.strip()} <{sender_email.strip()}>"
-    message["To"] = recipient.strip()
-    message["Subject"] = f"SEPA-Lastschrift C3MA – {collection_date.strftime('%d.%m.%Y')}"
-    message["X-Mozilla-Draft-Info"] = (
-        "internal/draft; vcard=0; receipt=0; DSN=0; uuencode=0; attachmentreminder=0"
+    values = template_values(member, organization, signature=signature, created_at=created_at)
+    values.update(
+        {
+            "debit.amount": f"{debit.amount:.2f}",
+            "debit.collection_date": collection_date.strftime("%d.%m.%Y"),
+            "debit.purpose": debit.purpose,
+            "debit.mandate_reference": debit.mandate_reference,
+        }
     )
-    greeting = first_name.strip() or "Mitglied"
-    message.set_content(
-        "\n".join(
-            (
-                f"Hallo {greeting},",
-                "",
-                "hiermit informieren wir dich, dass wir den folgenden Betrag per "
-                "SEPA-Lastschrift einziehen werden:",
-                "",
-                f"Betrag: {debit.amount:.2f} Euro",
-                f"Einzugsdatum: {collection_date.strftime('%d.%m.%Y')}",
-                f"Verwendungszweck: {debit.purpose}",
-                f"Mandatsreferenz: {debit.mandate_reference}",
-                f"Gläubiger-Identifikationsnummer: {creditor_id}",
-                "",
-                "Sollten die Angaben nicht stimmen, melde dich bitte zeitnah bei uns.",
-                "",
-                "Viele Grüße",
-                signature.strip(),
-            )
-        ),
-        charset="utf-8",
+    rendered = render_template(repository, "sepa", values)
+    return compose_mail(
+        recipient=member.email,
+        subject=rendered.subject,
+        body=rendered.body,
+        sender_name=sender_name,
+        sender_email=sender_email,
+        created_at=created_at,
     )
-    return message.as_bytes()
 
 
 def generate_debit_mails(
@@ -149,11 +128,11 @@ def generate_debit_mails(
                 warnings.append(f"{member.member_number or member.display_name}: E-Mail-Adresse fehlt.")
                 continue
             content = debit_mail_bytes(
-                recipient=member.email,
-                first_name=member.first_name,
+                repository,
+                member=member,
                 debit=debit,
                 collection_date=collection_date,
-                creditor_id=creditor_id,
+                organization=organization,
                 sender_name=sender_name,
                 sender_email=sender_email,
                 signature=signature,