feat(operator): adapt Slack personal polling cadence

[edwin-zvs]

Summary

• treat the configured Slack-personal poll interval as the idle ceiling
• reset accepted activity to the 5-second safe floor, then double idle waits back to the ceiling
• keep out-of-scope account traffic from holding the channel at the active cadence
• document the timing contract and clarify the TUI/web setting label

Timing semantics

• startup and backend reconnect: configured idle ceiling
• accepted ingress activity: next sweep in 5 seconds
• each idle sweep: double the next interval, capped at the configured ceiling
• default sequence after activity: 5s, 10s, 20s

Verification

• cargo test -p construct-daemon (725 passed)
• cargo test -p construct-cli slack_personal (1 passed)
• cargo build

No recording included: this is backend scheduling behavior; the only visible change is configuration copy clarifying the setting semantics.