Add --log-interval flag and timestamp capture logs

Follow-ups from the capture-reliability review:
- All server-side capture log lines are now prefixed with a local
  timestamp (YYYY-MM-DD HH:MM:SS) for easier correlation.
- New --log-interval SECONDS CLI flag (default 60) tunes the
  "Capture alive" heartbeat interval; passed through to CaptureManager.

Stays on 1.22.0 (backend-only). 369 tests pass.

README.md

@@ -113,12 +113,14 @@ simtrace2-pysniff-server --capture direct
 simtrace2-pysniff-server --capture disabled
 ```
 
-The server logs its capture liveness to stderr: `Capture started` on
-begin, a periodic `Capture alive: session=N messages=M bytes=B dropped=D`
-heartbeat, and `Capture stopped` (or `ERROR: capture thread stopped on
-error …` if the capture thread died unexpectedly). A mis-framed or
-non-SIM GSMTAP datagram is dropped and counted (`dropped`) rather than
-silently killing the capture.
+The server logs its capture liveness to stderr (each line timestamped
+with the local time): `Capture started` on begin, a periodic
+`Capture alive: session=N messages=M bytes=B dropped=D` heartbeat, and
+`Capture stopped` (or `ERROR: capture thread stopped on error …` if the
+capture thread died unexpectedly). A mis-framed or non-SIM GSMTAP
+datagram is dropped and counted (`dropped`) rather than silently
+killing the capture. The heartbeat interval defaults to 60 s and is
+tunable via `--log-interval SECONDS`.
 
 The GSMTAP listener also accepts the **sigrok-iso7816-stream** decoder
 (https://github.com/anttro/sigrok_iso7816_stream) — an FX2-logic-analyzer