One printer picker for machines and PCs, and one default per asset

The assignment belongs to the MACHINE, and until now there was no way to set it
except the generic relationships card or the API - the form for the thing the
feature is about did not exist. MachineForm now carries the picker, and PCForm
uses the SAME component rather than its own copy: the PC's set overrides the
machine's, and two implementations of that would drift, with the two ends of an
override disagreeing being exactly the bug nobody would spot.

The shared picker also fixes what PCForm did on save. It wrote row at a time
through the generic relationship endpoints, which is a non-atomic reconcile: an
HTTP failure part way left a PC half-assigned with nothing recording what was
meant. It now calls the reconcile endpoint, which validates the default before
writing anything.

A relationship type can now say it allows one active row per asset
(relationshiptypes.issingular, migration 7d34), and defaultprinter says it.
Cardinality belongs to the type rather than the printers plugin: core's create
path is where every hand-made link passes, and the next type meaning "exactly
one" gets the rule for free. Setting a second default REPLACES the first instead
of refusing, because "make this the default" means that - and a card answering
409 would leave the user hunting for the old row.

Without it the schema was happy to hold two defaults: the unique constraint is
(source, target, type), so two different targets are two valid rows, and the
resolver takes the OLDEST - the new default silently lost. Proven by disabling
the new rule and watching the tests fail.

FOUND WHILE TESTING IN A BROWSER, and it was not mine: MachineForm read
.data.data off computersApi.listAll(), which resolves to the ARRAY - fetchAllPages
has already unwrapped every page. The whole parallel load threw into the catch,
so every dropdown on the machine edit form came up empty and the machine's own
values never loaded. A build cannot see this; only opening the page can.

GET /api/printers/assignments/for-asset/<id> returns an asset's OWN assignment,
without inheritance, because the editor must show what this asset's rows say -
otherwise a machine's printers appear ticked on the PC that inherits them and
unticking one silently creates an override.

migrations/versions/7d34_singular_relationship_types.py

@@ -0,0 +1,46 @@
+"""Let a relationship type say "at most one of these per asset".
+
+A PC has one default printer. The unique constraint on assetrelationships is
+(source, target, type), which happily accepts two DIFFERENT defaults on one
+asset - and the resolver then takes the oldest, so setting a new default through
+the generic relationships card left the old one winning, silently.
+
+Cardinality belongs to the TYPE, not to the printers plugin: the next type that
+means "exactly one" (a primary user, a primary location) gets the rule for free,
+and core's create path is where every write already passes.
+
+Revision ID: 7d34_singular_relationship_types
+Revises: 7d33_buildings_and_levels
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = '7d34_singular_relationship_types'
+down_revision = '7d33_buildings_and_levels'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    bind = op.get_bind()
+    inspector = sa.inspect(bind)
+    columns = {column['name'] for column in inspector.get_columns('relationshiptypes')}
+    if 'issingular' not in columns:
+        op.add_column('relationshiptypes',
+                      sa.Column('issingular', sa.Boolean(), nullable=False,
+                                server_default=sa.false()))
+
+    # defaultprinter is the type this exists for, and it is already in use, so
+    # set it here rather than waiting for a re-seed: a site that upgrades and
+    # does not re-seed would otherwise keep the old silent behaviour.
+    op.execute("UPDATE relationshiptypes SET issingular = 1 "
+               "WHERE LOWER(relationshiptype) = 'defaultprinter'")
+
+
+def downgrade():
+    bind = op.get_bind()
+    inspector = sa.inspect(bind)
+    columns = {column['name'] for column in inspector.get_columns('relationshiptypes')}
+    if 'issingular' in columns:
+        op.drop_column('relationshiptypes', 'issingular')