Add the measuringtools plugin (ADR-005) and the plugin-system tutorial

Gage-lab instruments as Asset extensions: measuringtooltypes (color-coded
lookup) + measuringtools (calibration interval/dates, provider, notes) with
calibration status derived at read time (overdue / due soon / current /
unknown), never stored. Full CRUD API with permission-gated writes, types
management with in-use guard, calibration report, nav/reports/config-schema
hooks, and a complete frontend (list/detail/form, types settings page,
calibration report page, gated routes per ADR-009).

First plugin whose migration chain really creates tables post-cutover
(ADR-008), and the working example for docs/PLUGIN-GUIDE.md - a 12-section
walkthrough of building a plugin on this framework, linked from
PLUGIN-QUICKSTART and PLUGINS.

Verified: full suite 323 passing, live E2E on all four pages, fresh
scratch-MySQL migration dry-run green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

plugins/measuringtools/models/__init__.py

@@ -0,0 +1,17 @@
+"""Measuring-tools plugin models."""
+
+from .measuringtool import (
+    MeasuringTool,
+    MeasuringToolType,
+    derive_status,
+    STATUS_COLORS,
+    DUESOON_WINDOW_DAYS,
+)
+
+__all__ = [
+    'MeasuringTool',
+    'MeasuringToolType',
+    'derive_status',
+    'STATUS_COLORS',
+    'DUESOON_WINDOW_DAYS',
+]