feat(slice-2): platform/mailer port + LogMailer (dev channel, §6.2/PUC-10)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/platform/mailer.py

@@ -0,0 +1,50 @@
+"""platform/mailer — the outbound email port (SD-0001 §6.2).
+
+A one-method port `send(to, subject, body)` with two adapters: `LogMailer` (dev/tests —
+the message lands in the app log and an in-memory outbox tests read back, §6.8) and the
+deployed `SmtpMailer` (SLICE-4). The adapter is chosen by configuration at startup
+(INV-8), so no deployment shape lives in the domain. LogMailer logs the full body on
+purpose: that is PUC-10's "local dev channel" — the one-time code reaches the developer in
+the terminal. The deployed mailer (SLICE-4) will NOT log the body (INV-3 / §6.6 log hygiene).
+"""
+from __future__ import annotations
+
+import logging
+from dataclasses import dataclass
+from typing import Protocol
+
+logger = logging.getLogger("ecomm.mailer")
+
+
+@dataclass(frozen=True)
+class SentMessage:
+    to: str
+    subject: str
+    body: str
+
+
+class Mailer(Protocol):
+    """The outbound-message port. Raises on delivery failure (INV-9 → §6.4 502)."""
+
+    def send(self, to: str, subject: str, body: str) -> None: ...
+
+
+class LogMailer:
+    """Dev/test adapter: records to an in-memory outbox and logs the message (PUC-10)."""
+
+    def __init__(self) -> None:
+        self.outbox: list[SentMessage] = []
+
+    def send(self, to: str, subject: str, body: str) -> None:
+        self.outbox.append(SentMessage(to=to, subject=subject, body=body))
+        # Full body incl. the code — the local dev channel. Dev/test only (never deployed).
+        logger.info("LogMailer -> %s | %s\n%s", to, subject, body)
+
+
+def build_mailer(kind: str) -> Mailer:
+    """Select the mailer adapter by configured kind (config.mailer_kind(), INV-8)."""
+    if kind == "log":
+        return LogMailer()
+    if kind == "smtp":
+        raise NotImplementedError("SmtpMailer arrives in SLICE-4 (deployed environments)")
+    raise ValueError(f"unknown mailer kind: {kind!r}")