HeartbeatService.Start() always returned early because running()
checked stopChan closure state, which is "open" (= true) for a
newly created service. This caused Start() to interpret a fresh
service as "already running" and skip launching the goroutine.
Introduce a `started` bool field to separate "has been started"
from "has not been stopped", fixing both the start failure and
a potential double-close panic on Stop().
Run() and Stop() access the `running` field from different goroutines
without synchronization. Replace the bare `bool` with `sync/atomic.Bool`
to eliminate the data race.
Multiple packages had their own private truncate implementations:
- channels/telegram.go: truncateString (byte-based, no "...")
- channels/dingtalk.go: truncateStringDingTalk (byte-based, no "...")
- voice/transcriber.go: truncateText (byte-based, with "...")
All three are functionally equivalent to the existing utils.Truncate,
which already handles rune-safe truncation and appends "..." correctly.
Replace all private copies with utils.Truncate and delete the dead code.
Together