network: generate a device's asset number instead of asking twice

Every network device on this fleet already follows one convention, applied by
hand: AP-<name>, SW-<name>, SVR-<name>, IDF-<name>. 45 records, no exceptions.
The create form demanded the asset number anyway, so the same value was typed
twice and the convention held only as long as everyone remembered it.

The prefix now lives on the device type, and a blank asset number is generated
as <PREFIX>-<name>. Left explicit, an asset number always wins: a device
carrying a real identifier of its own - a vendor tag, a controller name, a
serial - keeps it. That is the platform rule, adopt where an identifier exists
and derive only where none does.

The prefix is NOT derived from the type name. "Access Point" and "Access Panel"
both initialise to AP, and assetnumber is unique, so the second type would
collide with the first on every device it created. It is nullable, so a type
that wants no prefix generates the bare name rather than needing one invented.

Names are sanitised before they reach a business key - the existing data
already shows why, with IDF-Telco-Demarc-#1 carrying a '#' into an identifier.
An existing prefix is never stacked: IDF-03 under type IDF stays IDF-03.

plugins/network/models/network_device.py

@@ -16,6 +16,16 @@ class NetworkDeviceType(BaseModel):
     description = db.Column(db.Text)
     icon = db.Column(db.String(50), comment='Icon name for UI')
     color = db.Column(db.String(20), comment='CSS color for UI/map markers')
+    # Asset-number prefix for devices of this type: AP-<name>, SW-<name>.
+    # NULLABLE on purpose - a type with no prefix generates the bare name, so
+    # nobody has to invent one for a type that does not want it.
+    #
+    # It cannot be derived from the type NAME: 'Access Point' and 'Access Panel'
+    # both initialise to AP, and assetnumber is unique, so the second type would
+    # collide with the first on every device.
+    prefix = db.Column(
+        db.String(12),
+        comment='Asset-number prefix for this type (AP, SW, SVR). Blank = none')
 
     def __repr__(self):
         return f"<NetworkDeviceType {self.networkdevicetype}>"