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/mail_composition.py

@@ -0,0 +1,94 @@
+from __future__ import annotations
+
+from datetime import datetime
+from email.message import EmailMessage
+from email.policy import SMTP
+from email.utils import format_datetime, make_msgid
+
+from ccma.domain.mail_templates import MailTemplate, MailTemplateError, render_mail_template
+from ccma.domain.models import Member
+from ccma.domain.placeholders import base_placeholder_values
+from ccma.storage.repository import MemberRepository, RepositoryError
+
+
+def template_values(
+    member: Member,
+    organization: dict | None,
+    *,
+    signature: str,
+    created_at: datetime | None = None,
+) -> dict[str, str]:
+    values = base_placeholder_values(member, organization, created_at=created_at)
+    # A mail always opens with a salutation, so an empty first name would read as
+    # "Hallo ," -- every other placeholder may legitimately render empty.
+    values["member.first_name"] = member.first_name.strip() or "Mitglied"
+    values["signature"] = signature.strip()
+    return values
+
+
+def payment_instructions(
+    member: Member,
+    organization: dict | None,
+    *,
+    due_date: str,
+    reference: str,
+) -> str:
+    """Ready-made payment paragraph: members with an active mandate are told the
+    money is collected, everyone else gets the club's bank details."""
+    organization = organization if isinstance(organization, dict) else {}
+    if member.mandate_active:
+        mandate = member.mandate_reference.strip()
+        mandate_hint = f" (Mandatsreferenz {mandate})" if mandate else ""
+        return (
+            f"Du hast uns ein SEPA-Lastschriftmandat erteilt{mandate_hint} – wir ziehen den "
+            "Betrag fristgerecht von deinem Konto ein. Du musst also nichts weiter tun; "
+            "vor dem Einzug informieren wir dich rechtzeitig per E-Mail."
+        )
+    deadline = f" bis zum {due_date}" if due_date else ""
+    lines = [f"Bitte überweise den Betrag{deadline} auf unser Vereinskonto:", ""]
+    lines.append(f"IBAN: {str(organization.get('iban', '')).strip()}")
+    bic = str(organization.get("bic", "")).strip()
+    if bic:
+        lines.append(f"BIC: {bic}")
+    if reference.strip():
+        lines.append(f"Verwendungszweck: {reference.strip()}")
+    return "\n".join(lines)
+
+
+def render_template(
+    repository: MemberRepository,
+    key: str,
+    values: dict[str, str],
+    repeats: dict[str, list[dict[str, str]]] | None = None,
+) -> MailTemplate:
+    try:
+        return render_mail_template(repository.get_mail_template(key), values, repeats)
+    except MailTemplateError as exc:
+        raise RepositoryError(str(exc)) from exc
+
+
+def compose_mail(
+    *,
+    recipient: str,
+    subject: str,
+    body: str,
+    sender_name: str,
+    sender_email: 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 den Versand 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"] = subject.strip()
+    message["X-Mozilla-Draft-Info"] = (
+        "internal/draft; vcard=0; receipt=0; DSN=0; uuencode=0; attachmentreminder=0"
+    )
+    message.set_content(body.strip() + "\n", charset="utf-8")
+    return message.as_bytes()