✨ feat(email): pkg/email + Mailpit docker-compose service (ADR-0029 Phase A.1)

Foundation for the passwordless auth migration (ADR-0028 Phase A) and
the BDD email-parallel strategy (ADR-0030). This PR ships only the
infrastructure — no auth code yet ; that lands in subsequent PRs.

Changes:
- docker-compose.yml: add mailpit service (axllent/mailpit:latest), SMTP
  on :1025, HTTP UI/API on :8025, MP_MAX_MESSAGES=5000
- pkg/email/sender.go: provider-agnostic Sender interface + Message struct
- pkg/email/smtp_sender.go: SMTPSender implementation (net/smtp), with
  Mailpit-friendly defaults (localhost:1025, no TLS, no AUTH), context-
  aware Send with timeout, supports plain text and multipart/alternative
- pkg/config: AuthConfig.Email field + EmailConfig struct + GetEmailConfig
  getter + 7 new env vars (DLC_AUTH_EMAIL_*) + defaults
- documentation/EMAIL.md: setup, inspection (UI + API), code examples,
  cross-refs to ADR-0028/0029/0030

Tests (pkg/email/smtp_sender_test.go):
- validateMessage rejects missing fields, accepts minimal
- buildRFC5322 plain-text path produces single-part text/plain with
  expected headers
- buildRFC5322 multipart path produces multipart/alternative with both
  parts and a closing boundary
- buildRFC5322 custom headers are canonicalised (lowercase keys → Title-Case)
- NewSMTPSender defaults are Mailpit-friendly
- Send respects context cancellation (no 10s wait when ctx cancelled)

Race detector clean. Build clean. Vet clean.

Out of scope for this PR (Phase A.2+):
- BDD email-steps helper package (pkg/bdd/mailpit/, pkg/bdd/steps/email_steps.go)
- magic_link_tokens table + repository
- magic-link/request and magic-link/consume HTTP handlers
- BDD scenarios for the magic-link flow

pkg/config/config.go

@@ -104,9 +104,22 @@ type APIConfig struct {
 
 // AuthConfig holds authentication configuration
 type AuthConfig struct {
-	JWTSecret           string    `mapstructure:"jwt_secret"`
-	AdminMasterPassword string    `mapstructure:"admin_master_password"`
-	JWT                 JWTConfig `mapstructure:"jwt"`
+	JWTSecret           string      `mapstructure:"jwt_secret"`
+	AdminMasterPassword string      `mapstructure:"admin_master_password"`
+	JWT                 JWTConfig   `mapstructure:"jwt"`
+	Email               EmailConfig `mapstructure:"email"`
+}
+
+// EmailConfig holds outgoing email transport configuration.
+// Defaults match local Mailpit (cf. ADR-0029) so dev needs no extra setup.
+type EmailConfig struct {
+	From         string        `mapstructure:"from"`
+	SMTPHost     string        `mapstructure:"smtp_host"`
+	SMTPPort     int           `mapstructure:"smtp_port"`
+	SMTPUsername string        `mapstructure:"smtp_username"`
+	SMTPPassword string        `mapstructure:"smtp_password"`
+	SMTPUseTLS   bool          `mapstructure:"smtp_use_tls"`
+	Timeout      time.Duration `mapstructure:"timeout"`
 }
 
 // JWTConfig holds JWT-specific configuration
@@ -256,6 +269,13 @@ func LoadConfig() (*Config, error) {
 	v.SetDefault("auth.jwt.secret_retention.max_retention", 72*time.Hour)
 	v.SetDefault("auth.jwt.secret_retention.cleanup_interval", 1*time.Hour)
 
+	// Email defaults — match local Mailpit (ADR-0029).
+	v.SetDefault("auth.email.from", "noreply@dance-lessons-coach.local")
+	v.SetDefault("auth.email.smtp_host", "localhost")
+	v.SetDefault("auth.email.smtp_port", 1025)
+	v.SetDefault("auth.email.smtp_use_tls", false)
+	v.SetDefault("auth.email.timeout", 10*time.Second)
+
 	// Check for custom config file path via environment variable
 	if configFile := os.Getenv("DLC_CONFIG_FILE"); configFile != "" {
 		v.SetConfigFile(configFile)
@@ -301,6 +321,13 @@ func LoadConfig() (*Config, error) {
 	v.BindEnv("auth.jwt.secret_retention.retention_factor", "DLC_AUTH_JWT_SECRET_RETENTION_FACTOR")
 	v.BindEnv("auth.jwt.secret_retention.max_retention", "DLC_AUTH_JWT_SECRET_MAX_RETENTION")
 	v.BindEnv("auth.jwt.secret_retention.cleanup_interval", "DLC_AUTH_JWT_SECRET_CLEANUP_INTERVAL")
+	v.BindEnv("auth.email.from", "DLC_AUTH_EMAIL_FROM")
+	v.BindEnv("auth.email.smtp_host", "DLC_AUTH_EMAIL_SMTP_HOST")
+	v.BindEnv("auth.email.smtp_port", "DLC_AUTH_EMAIL_SMTP_PORT")
+	v.BindEnv("auth.email.smtp_username", "DLC_AUTH_EMAIL_SMTP_USERNAME")
+	v.BindEnv("auth.email.smtp_password", "DLC_AUTH_EMAIL_SMTP_PASSWORD")
+	v.BindEnv("auth.email.smtp_use_tls", "DLC_AUTH_EMAIL_SMTP_USE_TLS")
+	v.BindEnv("auth.email.timeout", "DLC_AUTH_EMAIL_TIMEOUT")
 	v.BindEnv("telemetry.sampler.type", "DLC_TELEMETRY_SAMPLER_TYPE")
 	v.BindEnv("telemetry.sampler.ratio", "DLC_TELEMETRY_SAMPLER_RATIO")
 
@@ -432,6 +459,13 @@ func (c *Config) GetAdminMasterPassword() string {
 	return c.Auth.AdminMasterPassword
 }
 
+// GetEmailConfig returns the outgoing email transport configuration.
+// Defaults match local Mailpit (localhost:1025, no TLS, no auth) per
+// ADR-0029. Used by pkg/email.NewSMTPSender.
+func (c *Config) GetEmailConfig() EmailConfig {
+	return c.Auth.Email
+}
+
 // GetJWTTTL returns the JWT TTL
 func (c *Config) GetJWTTTL() time.Duration {
 	if c.Auth.JWT.TTL == 0 {