Feature/ntp probe addition

[sempervent]

Summary

This PR adds first-class NTP support to OpenSAMPL and hardens the local/demo experience around initialization, loading, and visualization.

At a high level, it introduces:

• an NTP vendor/probe family
• local and remote NTP collection support
• NTP-specific metrics and metadata handling
• database bootstrap seeding during init
• reference-safe dashboard/query updates for NTP-backed timing views
• documentation for the NTP extension path

What’s included

NTP vendor / probe support

• Adds an NTP vendor family using the existing OpenSAMPL extension model
• Supports remote NTP querying and local NTP inspection paths
• Adds ntp_metadata handling and NTP-specific probe loading behavior
• Adds NTP metrics such as jitter, delay, stratum, and sync health

Bootstrap / init fixes

• opensampl init now seeds required lookup/default tables instead of only creating schema
• Ensures baseline reference_type, reference, metric_type, and defaults rows exist
• Adds/uses the default UUID function needed by load paths

Dashboard and query hardening

• Fixes Grafana variable/query issues around text UUID handling
• Removes brittle empty-filter and varchar = uuid failure modes
• Updates timing dashboards to use reference-safe wording for NTP-backed demo paths
• Adds compact reference/source metadata tables to aid interpretation
• Keeps GNSS extensibility intact for future true GNSS-backed probe families

NTP jitter handling

• Fixes the empty jitter path for remote NTP collection
• When a direct measured jitter value is unavailable from a single remote response, stores a documented estimate/bound instead
• Clarifies this distinction in dashboard wording and docs

Docs

• Updates README/docs to reflect current init/bootstrap behavior
• Adds a detailed guide explaining how the NTP extension was added and integrated into OpenSAMPL
• Adds an unreleased 1.2.0 changelog entry

Why

Before this change:

• OpenSAMPL init created tables but did not seed required lookup/default rows
• NTP support was not available as a first-class probe family
• Grafana dashboards were brittle around variable typing and empty selections
• NTP-backed demo dashboards could imply GNSS truth where only configured reference semantics existed

After this change:

• NTP-backed timing data can be collected, loaded, and visualized cleanly
• init/bootstrap is deterministic for fresh databases
• dashboard behavior is safer and more honest
• the extension path is documented for future maintainers

Notes

• The dashboard wording uses Reference rather than GNSS where the current backing data is NTP-referenced rather than GNSS-grounded.
• GNSS-related backend concepts are preserved; this PR does not remove future support for true GNSS-backed probe families.
• Demo/appliance orchestration (custom DB image, continuous ingest service, mock NTP targets, etc.) remains outside this PR in the syncscope-at-home repo.

Validation

Verified locally with:

• uv run pytest
• uv run ruff check .
• fresh database init/bootstrap
• NTP collect/load path
• Grafana dashboard rendering with populated probe metadata and time-series rows