Add Mailer — pluggable email delivery (#166)

[gregwinn]

Summary

New Mailer module — Mailer.send/3,4 — with a config-selected transport. Sub-issue E (#166) of the auth epic (#161) and the prerequisite for account recovery (#167). Branched off develop (no stacking).

Config.put(:mailer, :transport, :http)
Config.put(:mailer, :api_key, System.get_env("SENDGRID_API_KEY"))
Config.put(:mailer, :from, "no-reply@myapp.com")

Mailer.send("user@example.com", "Welcome!", "Thanks for signing up.")
Mailer.send(email, "Reset your password", html_body, %{html: true})

Transports

Transport	Behavior
:http	POSTs to SendGrid's v3 API. Uses hackney directly (since winn_http doesn't expose request headers, needed for Authorization: Bearer) — no new deps.
:test	Captures messages in-process (Mailer.captured/0 / Mailer.clear/0) so flows are assertable without sending mail.

SMTP is intentionally deferred — it would pull in a new dependency (e.g. gen_smtp). Noted in the module and docs; use :http for now. (Satisfies the issue's "SMTP implemented or explicitly deferred with a note".)

Testing

9 tests: capture, opts (from/html), ordering, clear, no-transport / unknown-transport errors, http-without-api-key error, SendGrid payload shape (text + html), and an e2e codegen-resolution test (Mailer.send through the compiler). Full suite 757, only the pre-existing winn_sqlite_tests esqlite-NIF failure.

Docs

docs/stdlib.md gains a Mailer section (transports table, send, test helpers); CHANGELOG updated.

Part of #161. Closes #166 once merged. Unblocks #167 (email verification + password reset).

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