fix(auth): build password-reset links from an operator-set public address, never the Host header (M462 #4981)

buildResetURL used r.Host and r.TLS, so a forgot-password request with a
forged Host emailed the victim a real reset token on a link to the
attacker's server. Links now come only from network_settings.public_url
(migration 0062), and no reset email is sent while it is empty; the response
stays the same opaque 200 and the log says why.

The address is set on a new "Public address" card under Admin → Integrations,
which offers the page's own origin and warns while unset. PUT
/api/admin/network-settings takes either field alone, so the proxy card and
this one can't overwrite each other.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/dbq/models.go

@@ -430,6 +430,7 @@ type MissingReacquisition struct {
 type NetworkSetting struct {
 	ID               bool
 	TrustedProxyHops int32
+	PublicUrl        string
 }
 
 type PasswordReset struct {

internal/db/dbq/network_settings.sql.go

@@ -10,23 +10,34 @@ import (
 )
 
 const getNetworkSettings = `-- name: GetNetworkSettings :one
-SELECT id, trusted_proxy_hops FROM network_settings WHERE id = true
+SELECT id, trusted_proxy_hops, public_url FROM network_settings WHERE id = true
 `
 
 func (q *Queries) GetNetworkSettings(ctx context.Context) (NetworkSetting, error) {
 	row := q.db.QueryRow(ctx, getNetworkSettings)
 	var i NetworkSetting
-	err := row.Scan(&i.ID, &i.TrustedProxyHops)
+	err := row.Scan(&i.ID, &i.TrustedProxyHops, &i.PublicUrl)
+	return i, err
+}
+
+const updatePublicURL = `-- name: UpdatePublicURL :one
+UPDATE network_settings SET public_url = $1 WHERE id = true RETURNING id, trusted_proxy_hops, public_url
+`
+
+func (q *Queries) UpdatePublicURL(ctx context.Context, publicUrl string) (NetworkSetting, error) {
+	row := q.db.QueryRow(ctx, updatePublicURL, publicUrl)
+	var i NetworkSetting
+	err := row.Scan(&i.ID, &i.TrustedProxyHops, &i.PublicUrl)
 	return i, err
 }
 
 const updateTrustedProxyHops = `-- name: UpdateTrustedProxyHops :one
-UPDATE network_settings SET trusted_proxy_hops = $1 WHERE id = true RETURNING id, trusted_proxy_hops
+UPDATE network_settings SET trusted_proxy_hops = $1 WHERE id = true RETURNING id, trusted_proxy_hops, public_url
 `
 
 func (q *Queries) UpdateTrustedProxyHops(ctx context.Context, trustedProxyHops int32) (NetworkSetting, error) {
 	row := q.db.QueryRow(ctx, updateTrustedProxyHops, trustedProxyHops)
 	var i NetworkSetting
-	err := row.Scan(&i.ID, &i.TrustedProxyHops)
+	err := row.Scan(&i.ID, &i.TrustedProxyHops, &i.PublicUrl)
 	return i, err
 }

internal/db/migrations/0062_network_public_url.down.sql

@@ -0,0 +1 @@
+ALTER TABLE network_settings DROP COLUMN IF EXISTS public_url;

internal/db/migrations/0062_network_public_url.up.sql

@@ -0,0 +1,8 @@
+-- The address users reach Minstrel at, e.g. https://music.example.com.
+--
+-- Password-reset links used to be built from the request's Host header,
+-- which the requester controls: a forgot-password call with a forged Host
+-- would email the victim a real reset token on a link to the attacker's
+-- server. Links are now built only from this operator-set value, and none
+-- are sent while it is empty (M462 #4981).
+ALTER TABLE network_settings ADD COLUMN public_url text NOT NULL DEFAULT '';

internal/db/queries/network_settings.sql

@@ -3,3 +3,6 @@ SELECT * FROM network_settings WHERE id = true;
 
 -- name: UpdateTrustedProxyHops :one
 UPDATE network_settings SET trusted_proxy_hops = $1 WHERE id = true RETURNING *;
+
+-- name: UpdatePublicURL :one
+UPDATE network_settings SET public_url = $1 WHERE id = true RETURNING *;