Support fallback NTP servers for edge time sync (#79)

[ff225]

Summary

A remote NTP server (the public pool configured in settings.yaml) can carry more time-synchronization error than a server on the local network. This adds support for an ordered list of fallback NTP servers per transport, so a local server can be configured as primary with the existing remote pool kept as a fallback (or vice versa) -- reducing the chance that transient unreachability of one server degrades sync quality or forces a fall back to offset=0.0.

Changes

• New server.communication.ntp_sync module: shared sync_with_ntp_fallback (startup, retries every server each round with exponential backoff) and resync_ntp_fallback (periodic re-sync, tries each server once) helpers, extracted from the three near-identical implementations in http_server.py, websocket_server.py, and mqtt_client.py.
• All three transports accept an optional ntp_fallback_servers: list[str] constructor arg and try [ntp_server, *ntp_fallback_servers] in order instead of only the single configured server.
• src/sciot/config.py: new optional communication.{http,websocket,mqtt}.ntp_fallback_servers config key (list of non-empty strings), validated via a new _optional_string_list helper.
• src/server/edge/run_edge.py: wires the new config key through to each transport constructor.
• src/server/settings.yaml: documents the new key with a commented-out example under each transport's ntp_server.

Backwards compatible: the new parameter defaults to None/[] everywhere, so existing configs without ntp_fallback_servers behave exactly as before (single server, same retry/backoff schedule).

Test plan

• New tests/unit/test_ntp_sync.py: primary-server success, fallback on primary failure, all-servers-unreachable (0.0 offset), and periodic resync variants -- all with a mocked ntplib.NTPClient and mocked sleep, no real network.
• tests/unit/test_config_validation.py: new cases for invalid ntp_fallback_servers (wrong type / empty string) on each transport, and a valid-list acceptance test.
• Fast CI suite: uv run pytest tests/unit tests/integration/test_client_resilience.py tests/integration/test_client_lifecycle.py tests/integration/test_variance_and_local_inference.py tests/test_mqtt_client tests/test_offloading_algo -- 223 passed, 1 skipped.
• ruff check on all changed files -- clean.

Closes #79