feat(admin): search users by email + erase a bound email

Add an exact (strict) email filter to the /users list (UserFilter.EmailExact →
a kind='email' identity match) with a search input, and an 'Erase email' action on
the user card that deletes the bound email identity and its pending confirmations,
freeing the address. It refuses to remove the account's only identity
(ErrLastIdentity), which would leave it unreachable. Integration tests for both.

backend/internal/adminconsole/views.go

@@ -62,6 +62,7 @@ type UsersView struct {
 	Robots         bool
 	NameMask       string
 	ExternalIDMask string
+	EmailExact     string
 	FilterQuery    template.URL
 }
 
@@ -161,7 +162,9 @@ type UserDetailView struct {
 	HasStats     bool
 	Stats        StatsRow
 	Identities   []IdentityRow
-	Games        []GameRow
+	// HasEmail gates the "Erase email" action; set when the account carries an email identity.
+	HasEmail bool
+	Games    []GameRow
 	// TelegramID and VKID are the account's platform external ids (empty when absent).
 	// TelegramID gates the "Send Telegram message" operator action; VKID surfaces the VK
 	// user id with a link to the VK profile (there is no VK messaging to drive).