feat(04-03): select memory, log, and SMTP mail drivers from config

- Load mail.* and SUMMER_MAIL__ overrides to pick memory, log, or smtp
- Log headers and text without credentials; SMTP uses explicit TLS and go-mail
- Reject CR/LF headers and invalid addresses and surface driver errors once

Co-authored-by: Cursor <cursoragent@cursor.com>

postcard/mailer.go

@@ -3,9 +3,12 @@ package postcard
 import (
 	"context"
 	"fmt"
+	"log/slog"
 	"strings"
+	"time"
 
 	"git.golem15.com/golem15/summercms/backpack"
+	"git.golem15.com/golem15/summercms/compass"
 	"git.golem15.com/golem15/summercms/pact"
 )
 
@@ -69,6 +72,10 @@ func (m *mailer) Send(ctx context.Context, msg Message) error {
 		return err
 	}
 	rendered.From = m.opts.From
+	_, smtp := m.driver.(*SMTPDriver)
+	if err := validateRendered(rendered, smtp); err != nil {
+		return err
+	}
 	if err := m.driver.Send(ctx, rendered); err != nil {
 		return fmt.Errorf("postcard: %w", err)
 	}
@@ -81,15 +88,13 @@ func Activate[P interface{ ID() string }](app *backpack.App, _ []P) error {
 	if app == nil {
 		return fmt.Errorf("postcard: app is nil")
 	}
-	opts := Options{}
-	if app.Config != nil {
-		opts.From = strings.TrimSpace(app.Config.String("mail.from"))
-		opts.CSS = app.Config.String("mail.css")
-		opts.BrandCSS = app.Config.String("mail.brandCss")
+	driver, opts, err := driverFromApp(app)
+	if err != nil {
+		return err
 	}
 	m := &mailer{
 		catalog: NewCatalog(),
-		driver:  NewMemoryDriver(),
+		driver:  driver,
 		opts:    opts,
 	}
 	if err := app.Publish[Mailer](m); err != nil {
@@ -98,6 +103,64 @@ func Activate[P interface{ ID() string }](app *backpack.App, _ []P) error {
 	return nil
 }
 
+func driverFromApp(app *backpack.App) (Driver, Options, error) {
+	opts := Options{}
+	name := "memory"
+	smtp := SMTPConfig{Port: 587, Timeout: 10 * time.Second}
+	if app != nil && app.Config != nil {
+		c := app.Config
+		if v := strings.TrimSpace(c.String("mail.driver")); v != "" {
+			name = strings.ToLower(v)
+		}
+		opts.From = strings.TrimSpace(c.String("mail.from"))
+		opts.CSS = c.String("mail.css")
+		opts.BrandCSS = c.String("mail.brandCss")
+		smtp = smtpFromConfig(c)
+	}
+	switch name {
+	case "memory":
+		return NewMemoryDriver(), opts, nil
+	case "log":
+		return NewLogDriver(loggerFromApp(app)), opts, nil
+	case "smtp":
+		d, err := NewSMTPDriver(smtp)
+		if err != nil {
+			return nil, opts, err
+		}
+		return d, opts, nil
+	default:
+		return nil, opts, fmt.Errorf("postcard: unknown mail.driver %q (use memory, log, or smtp)", name)
+	}
+}
+
+func smtpFromConfig(c *compass.Config) SMTPConfig {
+	cfg := SMTPConfig{
+		Host:     strings.TrimSpace(c.String("mail.smtp.host")),
+		Port:     c.Int("mail.smtp.port"),
+		Username: c.String("mail.smtp.username"),
+		Password: c.String("mail.smtp.password"),
+		TLS:      strings.TrimSpace(c.String("mail.smtp.tls")),
+		Timeout:  10 * time.Second,
+	}
+	if raw := strings.TrimSpace(c.String("mail.smtp.timeout")); raw != "" {
+		if d, err := time.ParseDuration(raw); err == nil {
+			cfg.Timeout = d
+		} else if n := c.Int("mail.smtp.timeout"); n > 0 {
+			cfg.Timeout = time.Duration(n) * time.Second
+		}
+	}
+	return cfg
+}
+
+func loggerFromApp(app *backpack.App) *slog.Logger {
+	if app != nil {
+		if log, ok := app.Lookup[*slog.Logger](); ok && log != nil {
+			return log
+		}
+	}
+	return slog.Default()
+}
+
 // BootPlugin registers a plugin's declared mail templates and layouts.
 // Missing files and unknown layout aliases fail with the dotted name.
 func BootPlugin[P interface{ ID() string }](app *backpack.App, p P) error {