Implement the birthday wisher core (MIL-002)

Layered package (domain, application, infrastructure, presentation), CLI with
--dry-run and --today, SMTP with STARTTLS behind a port, 56 offline tests and
the Python code review RC-006.

Task: MIL-002#1
Task: MIL-002#2
Task: MIL-002#3
Task: MIL-002#4
Task: MIL-002#5
Task: MIL-002#6
Refs #7
Refs #8
Refs #9
Refs #10
Refs #11
Refs #12

tests/integration/infrastructure/test_letter_templates.py

@@ -0,0 +1,34 @@
+"""Integration tests for FolderLetterSource against real files."""
+
+from pathlib import Path
+
+import pytest
+
+from birthday_wisher import constants
+from birthday_wisher.application.use_cases.send_birthday_wishes import build_letter
+from birthday_wisher.domain.errors import LetterTemplateError
+from birthday_wisher.infrastructure.letter_templates import FolderLetterSource
+
+LETTERS_DIR = Path(__file__).resolve().parents[3] / constants.LETTER_TEMPLATES_DIR
+
+
+@pytest.mark.parametrize("number", range(1, constants.LETTER_COUNT + 1))
+def test_every_real_template_can_be_chosen_and_built(number: int) -> None:
+    """Each shipped template holds the placeholder and is fully replaced."""
+    template = FolderLetterSource(LETTERS_DIR).read_letter(number)
+    assert constants.NAME_PLACEHOLDER in template
+    letter = build_letter(template, "Zelda")
+    assert "Zelda" in letter
+    assert constants.NAME_PLACEHOLDER not in letter
+
+
+def test_read_letter_reads_file_from_folder(tmp_path: Path) -> None:
+    """A template file is returned as written."""
+    (tmp_path / "letter_1.txt").write_text("Hi [NAME]", encoding="utf-8")
+    assert FolderLetterSource(tmp_path).read_letter(1) == "Hi [NAME]"
+
+
+def test_read_letter_missing_file_raises_template_error(tmp_path: Path) -> None:
+    """A missing template raises LetterTemplateError."""
+    with pytest.raises(LetterTemplateError):
+        FolderLetterSource(tmp_path).read_letter(1)