feat: provider source for oauth-providers and sign-in helpers for social login

- classes.OAuthProvider and classes.OAuthProviderSource; GET /oauth-providers
  looks the source up per request and keeps the empty body without one
- exported RegistrationSettings, RegistrationsFromIP, RefreshTTL, RemoteIP
- RegisterOptions.PasswordLess (has_self_set_password=false) and FinishLogin
  (restore + reactivate mail, last_ip_address, last_seen; no event)
- TestOAuthProvidersSourceSmoke, TestSocialHooksSmoke; README

controllers/registration.go

@@ -3,6 +3,7 @@ package controllers
 import (
 	"context"
 	"errors"
+	"log/slog"
 	"maps"
 	"net/http"
 	"time"
@@ -26,6 +27,11 @@ import (
 type RegisterOptions struct {
 	Activate bool
 	IP       string
+	// PasswordLess creates the account with has_self_set_password=false: its
+	// password is random and was never known to the user (social sign-in,
+	// PHP password_bootstrap_source oauth_registration). The zero value
+	// keeps true.
+	PasswordLess bool
 }
 
 // RegisterUser validates fields against the user model's rules, hashes the
@@ -68,7 +74,7 @@ func RegisterUser(ctx context.Context, app *backpack.App, db *gorm.DB, fields ma
 	if err := lagoon.Fill(user, user.Fillable(), input, production); err != nil {
 		return nil, nil, err
 	}
-	user.HasSelfSetPassword = true
+	user.HasSelfSetPassword = !opts.PasswordLess
 	if opts.Activate {
 		user.IsActivated = true
 		now := time.Now()
@@ -83,6 +89,40 @@ func RegisterUser(ctx context.Context, app *backpack.App, db *gorm.DB, fields ma
 	return user, nil, nil
 }
 
+// FinishLogin runs the steps password Login takes after it has
+// authenticated a user, for a caller that authenticated the user another way
+// (a social sign-in). It restores a soft-deleted account and sends the
+// reactivate mail, records ip as last_ip_address, and touches last_seen. It
+// checks no password, throttle, activation or ban and fires no event; the
+// caller does those checks (Go has no login event). A failed last_seen
+// write is logged and does not fail it, as in Login.
+func FinishLogin(ctx context.Context, app *backpack.App, db *gorm.DB, user *models.User, ip string) error {
+	if db == nil {
+		return errors.New("golem15.user: gorm handle is missing")
+	}
+	if user == nil || user.ID == 0 {
+		return errors.New("golem15.user: finishing a login needs a stored user")
+	}
+	if user.DeletedAt.Valid {
+		if err := db.WithContext(ctx).Unscoped().Model(user).Update("deleted_at", nil).Error; err != nil {
+			return err
+		}
+		user.DeletedAt = gorm.DeletedAt{}
+		sendTemplateVars(ctx, app, mailName(ctx, user, "golem15.user::mail.reactivate"), user.Email, mailVars(displayName(user), "", ""))
+	}
+	if ip != "" {
+		if err := db.WithContext(ctx).Unscoped().Model(user).Update("last_ip_address", ip).Error; err != nil {
+			return err
+		}
+		addr := ip
+		user.LastIPAddress = &addr
+	}
+	if err := classes.TouchLastSeen(ctx, db, user.ID); err != nil {
+		slog.Default().Warn("golem15.user: stamp last_seen on login", "user_id", user.ID, "err", err)
+	}
+	return nil
+}
+
 // FireRegisterEvent fires classes.RegisterEvent for user with Payload set
 // to a copy of fields without password and password_confirmation, and
 // returns the listeners' error. /register ignores that error, as Winter