feat(core): bind request services and implement registered tasks

Wire plugin services through Bind/InjectFrom and AppContext so HTTP and
workers resolve dependencies after Apply. Register TaskHandler objects
with persisted results, and implement send_email_code, mail:send,
cleanup_inactive_users, and dispatch_bot_msg.

backend/core/appctx.go

@@ -0,0 +1,43 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package core
+
+import "context"
+
+type appContextKey struct{}
+
+// WithAppContext attaches the micro-kernel Context to a standard context.Context
+// so request and worker handlers can Inject services without package-level setters.
+func WithAppContext(ctx context.Context, app *Context) context.Context {
+	if ctx == nil {
+		ctx = context.Background()
+	}
+	if app == nil {
+		return ctx
+	}
+	return context.WithValue(ctx, appContextKey{}, app.Root())
+}
+
+// AppContext extracts the micro-kernel Context from ctx, if present.
+func AppContext(ctx context.Context) *Context {
+	if ctx == nil {
+		return nil
+	}
+	if c, ok := ctx.(*Context); ok {
+		return c
+	}
+	app, _ := ctx.Value(appContextKey{}).(*Context)
+	return app
+}
+
+// InjectFrom resolves T from ctx when it carries a micro-kernel Context
+// (*Context itself, or a value attached by WithAppContext).
+func InjectFrom[T any](ctx context.Context) (T, error) {
+	var zero T
+	app := AppContext(ctx)
+	if app == nil {
+		return zero, ErrNilContext
+	}
+	return Inject[T](app)
+}