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/storage/atomic.py

@@ -63,6 +63,20 @@ def write_json_atomic(path: Path, data: Any) -> None:
         temporary.unlink(missing_ok=True)
 
 
+def write_text_atomic(path: Path, content: str) -> None:
+    path.parent.mkdir(parents=True, exist_ok=True)
+    descriptor, temporary_name = tempfile.mkstemp(prefix=f".{path.name}.", suffix=".tmp", dir=path.parent)
+    temporary = Path(temporary_name)
+    try:
+        with os.fdopen(descriptor, "w", encoding="utf-8", newline="\n") as handle:
+            handle.write(content)
+            handle.flush()
+            os.fsync(handle.fileno())
+        os.replace(temporary, path)
+    finally:
+        temporary.unlink(missing_ok=True)
+
+
 def read_json(path: Path) -> Any:
     with path.open("r", encoding="utf-8") as handle:
         return json.load(handle)