Collector: machine number identifies the machine, not the PC; ntlars empty-value fix

CHANGELOG.md

@@ -12,6 +12,19 @@ ADR-007 and ADR-002.
 
 ### Fixed
 
+- A shop-floor PC that reported the machine number of a machine ShopDB already
+  knew got a 500 from the collector, on every report, forever. The reported
+  machine number was being written to the PC's own `assets.assetnumber`, which
+  is uniquely indexed and already held by the machine, so the insert failed with
+  a duplicate-key error and the whole report was lost: operating system, boot
+  time, applications, printers and access protocols never landed. The machine
+  number now identifies the machine only. A new PC takes its hostname as its
+  asset number, which is what the existing data already does, and an existing
+  PC's asset number is never overwritten.
+- A backup export containing a value with an empty right-hand side (`Name=`)
+  failed to parse, and with it the whole file, so that machine could never be
+  backed up. The form is not strictly legal but occurs in real exports. It is
+  now read as an empty string, preserving the value name.
 - The 3D parts kiosk label prefix never appeared on the kiosk. The kiosk runs
   logged out, and an unauthenticated read of a setting is limited to an
   allowlist the key was not on, so the kiosk got a 404 and fell back to no
@@ -35,6 +48,17 @@ ADR-007 and ADR-002.
 
 ### Added
 
+- A PC reporting a machine number is now linked to that machine. Until now the
+  number was collected and then discarded, so a replaced PC never took over its
+  bay: the retired PC kept the link and everything that walks PC to machine, the
+  warranty machine column and the DNC info card among them, pointed at hardware
+  that had been pulled out. The link is a `controls` relationship carrying a
+  `collector:machine` origin label, so a link made by hand is never touched.
+  Superseding a PC archives the old link rather than deleting it, keeping the
+  history of which PC ran a machine and when, and raises an alert by email and
+  webhook. The retired PC's status is deliberately left alone: the collector
+  cannot tell whether it was shelved, sent for repair or re-imaged for another
+  bay, so a person decides.
 - Plugin contract 0.16.0: `get_settings_defaults()` lets a plugin declare the
   settings it owns (key, default, type, category, description, and whether an
   unauthenticated caller may read it). See `docs/PLUGIN-HOOKS.md`.