feat(gmail): the OAuth client can be entered instead of deployed

The one click that opens Google's consent screen already existed. What it needs
behind it is a registered client, and no app can conjure one: Google issues no
Gmail access to an unregistered client, and `gmail.readonly` is a restricted
scope, so an id shipped inside a public image would be both extractable from it
and unusable without Google's security review of every deployment using it.

The registration is a one-off. Having to edit a Rune's manifest and recreate the
container to paste the result was not.

`VERDANDE_GMAIL_CLIENT_ID` and `_SECRET` still win when set. A value in the
environment is a deliberate deployment decision, applied again on every recreate,
and a form that could quietly override it would make the manifest a lie — so the
page says where the value came from, and PUT answers 409.

The redirect URI is spelled out on the page rather than left to be reconstructed.
It is derived from VERDANDE_BASE_URL, it has to match Google's copy exactly, and
the error Google returns when it does not names neither value.

Its own table rather than `user_settings`: an OAuth client is the instance's, not
one person's — and `user_settings` has `user_id` as its whole primary key, so a
second scope there would silently overwrite the AI provider's key.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/httpapi/gmail_handlers.go

@@ -10,12 +10,44 @@ import (
 	"github.com/kristianwind/verdande/internal/store"
 )
 
-func (s *Server) gmailConfig() gmail.Config {
-	return gmail.Config{
+// gmailConfig is the OAuth client this instance authorises with.
+//
+// The environment wins when it is set. A value in the environment is a deliberate
+// deployment decision — it is in the Rune's manifest, under version control,
+// applied on every recreate — and a form that could quietly override it would make
+// the manifest a lie. The stored pair is for the ordinary case, where nobody wants
+// to redeploy a container to paste a client id.
+func (s *Server) gmailConfig(ctx context.Context) gmail.Config {
+	cfg := gmail.Config{
 		ClientID:     s.cfg.GmailClientID,
 		ClientSecret: s.cfg.GmailClientSecret,
 		RedirectURL:  s.cfg.GmailRedirectURL(),
 	}
+	if cfg.ClientID != "" && cfg.ClientSecret != "" {
+		return cfg
+	}
+
+	stored, err := s.db.InstanceSettings(ctx, "gmail")
+	if err != nil {
+		// Not fatal: an unconfigured Gmail and a database that could not answer
+		// look the same from here, and the caller's next step is the same either
+		// way — say it is not set up.
+		return cfg
+	}
+	if id, _ := stored["client_id"].(string); cfg.ClientID == "" {
+		cfg.ClientID = id
+	}
+	if secret, _ := stored["client_secret"].(string); cfg.ClientSecret == "" {
+		cfg.ClientSecret = secret
+	}
+	return cfg
+}
+
+// gmailFromEnv says whether the deployment set the client, which the settings page
+// needs so it can explain why the fields are not editable rather than accepting
+// changes that will never take effect.
+func (s *Server) gmailFromEnv() bool {
+	return s.cfg.GmailClientID != "" && s.cfg.GmailClientSecret != ""
 }
 
 // handleGmailAuthorize starts the flow: it mints a PKCE pair, remembers it against
@@ -27,7 +59,7 @@ func (s *Server) gmailConfig() gmail.Config {
 // defaults.
 func (s *Server) handleGmailAuthorize(w http.ResponseWriter, r *http.Request) {
 	user := userFrom(r.Context())
-	cfg := s.gmailConfig()
+	cfg := s.gmailConfig(r.Context())
 
 	if !cfg.Configured() {
 		writeError(w, http.StatusConflict, CodeGmailNotConfigured,
@@ -102,7 +134,7 @@ func (s *Server) handleGmailCallback(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	token, err := s.gmailConfig().Exchange(r.Context(), code, gmail.PKCE{Verifier: verifier})
+	token, err := s.gmailConfig(r.Context()).Exchange(r.Context(), code, gmail.PKCE{Verifier: verifier})
 	if err != nil {
 		s.log.Error("gmail exchange", "err", err, "user", user.ID)
 		http.Redirect(w, r, settingsURL+"?gmail=failed", http.StatusFound)
@@ -185,7 +217,7 @@ func (s *Server) SyncGmail(ctx context.Context, user *store.User) (int, error) {
 	access := str("access_token")
 	expires, _ := settings["expires_at"].(float64)
 	if access == "" || int64(expires) <= time.Now().Unix() {
-		token, err := s.gmailConfig().Refresh(ctx, refresh)
+		token, err := s.gmailConfig(ctx).Refresh(ctx, refresh)
 		if err != nil {
 			return 0, err
 		}
@@ -300,3 +332,96 @@ func (s *Server) handleVersion(w http.ResponseWriter, r *http.Request) {
 	}
 	writeJSON(w, http.StatusOK, status)
 }
+
+// --- the OAuth client itself ------------------------------------------------------
+
+// The registration Google requires, enterable rather than deployed.
+//
+// The one click that opens Google's consent screen already exists; what it needs
+// behind it is a registered client, and that is not something any app can conjure:
+// Google issues no Gmail access to an unregistered client, and `gmail.readonly` is
+// a restricted scope, so an id shipped inside a public image would be both
+// extractable from it and unusable without Google's security review of every
+// deployment. The registration is a one-off. Having to edit a Rune's manifest and
+// recreate the container to paste the result was not.
+//
+// Administrators only, and sessions only: this is the instance's identity to
+// Google, and a leaked API token must not be able to read or replace it.
+
+type gmailClientJSON struct {
+	ClientID string `json:"client_id"`
+	// Secret is written and never read back, like the AI key: a settings page that
+	// repopulates a password field is one that will eventually leak into a
+	// screenshot.
+	ClientSecret string `json:"client_secret,omitempty"`
+	HasSecret    bool   `json:"has_secret"`
+	// FromEnv says the deployment set this, in which case the stored pair is
+	// ignored and the fields are not worth editing. Sent so the page can say why
+	// rather than accepting changes that will never take effect.
+	FromEnv bool `json:"from_env"`
+	// RedirectURI is what has to be registered with Google, spelled out. It is
+	// derived from VERDANDE_BASE_URL and must match exactly — it is the single
+	// most likely thing to be wrong, and the error Google gives for it names
+	// neither value.
+	RedirectURI string `json:"redirect_uri"`
+}
+
+func (s *Server) handleGetGmailClient(w http.ResponseWriter, r *http.Request) {
+	stored, err := s.db.InstanceSettings(r.Context(), "gmail")
+	if err != nil {
+		s.internal(w, r, "gmail client", err)
+		return
+	}
+	out := gmailClientJSON{
+		FromEnv:     s.gmailFromEnv(),
+		RedirectURI: s.cfg.GmailRedirectURL(),
+	}
+	if out.FromEnv {
+		out.ClientID = s.cfg.GmailClientID
+		out.HasSecret = true
+	} else {
+		out.ClientID, _ = stored["client_id"].(string)
+		secret, _ := stored["client_secret"].(string)
+		out.HasSecret = secret != ""
+	}
+	writeJSON(w, http.StatusOK, out)
+}
+
+func (s *Server) handleSetGmailClient(w http.ResponseWriter, r *http.Request) {
+	var req gmailClientJSON
+	if err := decodeJSON(w, r, &req); err != nil {
+		return
+	}
+	if s.gmailFromEnv() {
+		writeError(w, http.StatusConflict, CodeConflict,
+			"the Gmail client is set in the environment and cannot be changed here")
+		return
+	}
+
+	stored, err := s.db.InstanceSettings(r.Context(), "gmail")
+	if err != nil {
+		s.internal(w, r, "gmail client", err)
+		return
+	}
+	// An empty secret means "leave it alone", not "delete it" — otherwise
+	// correcting a typo in the client id would silently clear the secret.
+	secret, _ := stored["client_secret"].(string)
+	if req.ClientSecret != "" {
+		secret = strings.TrimSpace(req.ClientSecret)
+	}
+	id := strings.TrimSpace(req.ClientID)
+
+	if fields := map[string]string{}; id == "" && secret != "" {
+		fields["client_id"] = "required"
+		writeFieldErrors(w, fields)
+		return
+	}
+
+	if err := s.db.SetInstanceSettings(r.Context(), "gmail", map[string]any{
+		"client_id": id, "client_secret": secret,
+	}); err != nil {
+		s.internal(w, r, "save gmail client", err)
+		return
+	}
+	w.WriteHeader(http.StatusNoContent)
+}