Add transactional email system via SMTP

Introduce internal/email package with SMTP sending, embedded HTML/text
templates, and multipart MIME assembly. Emails use a generic EmailData
struct (recipient name, message, optional button, optional closing) so
new email types can be added without code changes.

Wired into signup (welcome email), team creation, and team member
addition. No-op mailer when SMTP_HOST is not configured.

internal/api/handlers_team.go

@@ -1,6 +1,7 @@
 package api
 
 import (
+	"context"
 	"fmt"
 	"log/slog"
 	"net/http"
@@ -10,6 +11,7 @@ import (
 	"github.com/go-chi/chi/v5"
 	"github.com/jackc/pgx/v5/pgtype"
 
+	"git.omukk.dev/wrenn/wrenn/internal/email"
 	"git.omukk.dev/wrenn/wrenn/pkg/audit"
 	"git.omukk.dev/wrenn/wrenn/pkg/auth"
 	"git.omukk.dev/wrenn/wrenn/pkg/db"
@@ -18,12 +20,13 @@ import (
 )
 
 type teamHandler struct {
-	svc   *service.TeamService
-	audit *audit.AuditLogger
+	svc    *service.TeamService
+	audit  *audit.AuditLogger
+	mailer email.Mailer
 }
 
-func newTeamHandler(svc *service.TeamService, al *audit.AuditLogger) *teamHandler {
-	return &teamHandler{svc: svc, audit: al}
+func newTeamHandler(svc *service.TeamService, al *audit.AuditLogger, mailer email.Mailer) *teamHandler {
+	return &teamHandler{svc: svc, audit: al, mailer: mailer}
 }
 
 // teamResponse is the JSON shape for a team.
@@ -132,6 +135,15 @@ func (h *teamHandler) Create(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	go func() {
+		if err := h.mailer.Send(context.Background(), ac.Email, "Your team has been created", email.EmailData{
+			RecipientName: ac.Name,
+			Message:       fmt.Sprintf("Your team \"%s\" has been created on Wrenn. You can now invite members and start creating sandboxes under this team.", req.Name),
+		}); err != nil {
+			slog.Warn("failed to send team created email", "email", ac.Email, "error", err)
+		}
+	}()
+
 	writeJSON(w, http.StatusCreated, teamWithRoleResponse{
 		teamResponse: teamToResponse(team.Team),
 		Role:         team.Role,
@@ -280,6 +292,21 @@ func (h *teamHandler) AddMember(w http.ResponseWriter, r *http.Request) {
 	if parseErr == nil {
 		h.audit.LogMemberAdd(r.Context(), ac, targetUserID, member.Email, member.Role)
 	}
+
+	go func() {
+		team, err := h.svc.GetTeam(context.Background(), teamID)
+		teamName := "a team"
+		if err == nil {
+			teamName = team.Name
+		}
+		if err := h.mailer.Send(context.Background(), member.Email, "You've been added to a team on Wrenn", email.EmailData{
+			RecipientName: member.Name,
+			Message:       fmt.Sprintf("%s has added you to the team \"%s\" on Wrenn.", ac.Name, teamName),
+		}); err != nil {
+			slog.Warn("failed to send team invitation email", "email", member.Email, "error", err)
+		}
+	}()
+
 	writeJSON(w, http.StatusCreated, memberInfoToResponse(member))
 }