Add terminal and conductor tables to projectDataBase

Slice 2 of discussion #503 (from-to wiring list built on projectDataBase),
building on the conductor uuid from slice 1 (#625). Pure plumbing: two
new additive tables plus their populate/add/remove hooks. No view, no UI,
no visible behavior change yet -- the wiring-list view is slice 3.

Follows the existing shape of the class throughout: same table/column
naming, same prepared-statement idiom in prepareQuery(), same
bind/exec/qDebug-lastError error handling, same DELETE-then-loop
populate pattern.

- `terminal (uuid, element_uuid, name)` and
  `conductor (uuid, diagram_uuid, terminal1_uuid, terminal1_element_uuid,
   terminal2_uuid, terminal2_element_uuid, text)` created alongside the
  existing tables in createDataBase().
- populateConductorTable() added as a fifth populate* call in updateDB().
  Terminal population is folded into it, since a terminal only matters
  here in the context of a conductor referencing it.
- addConductor()/removeConductor() hooked into the already-existing
  Conductor::Type branch of Diagram::addItem()/removeItem(), mirroring
  the Element::Type branch directly above.

Two things the original schema sketch in the discussion got wrong, found
by testing rather than inspection:

1. Terminal::uuid() is NOT unique per placed terminal. It is the
   terminal-position id baked into the catalog .elmt definition ("the
   top terminal"), so every placed instance of the same catalog element
   shares it. A terminal instance is only uniquely identified by
   (uuid, element_uuid) together, so that pair is the terminal table's
   primary key and the conductor table carries both halves for each
   endpoint. With uuid alone as PK, the second placed instance of any
   element silently lost its terminals to the INSERT OR IGNORE.
2. Conductors whose terminals predate terminal uuids are omitted rather
   than given a fabricated identity, as agreed in the discussion. This
   turns out to matter far more than expected in practice -- see below.

Testing (all live, in the running app):

- Incremental add: fresh project, two vertically aligned contacts placed
  so autoconnect creates a conductor -> 2 terminals, 1 conductor.
- Incremental remove: deleting that conductor -> conductor count 1 -> 0.
- Undo: ctrl+Z after the delete -> back to 1, no duplicate-primary-key
  error (the same Conductor object keeps its uuid).
- Bulk populate: examples/weneedpolonez-Polonez_MR89_wiring_diagram.qet
  (366 conductors) -> 478 terminals, 280 conductors; the 86 conductors
  touching legacy terminals correctly omitted.
- Join correctness: conductor -> terminal (composite key) -> element_info
  resolves real from-to rows with real element labels.
- Legacy-only project: examples/industrial.qet has 1794 terminals and
  *zero* terminal uuids, so all 671 of its conductors are omitted. Loads
  and renders fine, no crash, no spurious rows -- but worth stating
  plainly that a from-to wiring list for that project would be empty
  today. This is a property of the element catalog definitions, not of
  the project file, and is the strongest argument for surfacing an
  "N conductors excluded" count to the user when the view lands.
- No SQL errors logged in any of the above.

Known limitation, consistent with existing behavior: removeDiagram()
does not cascade-delete the conductor rows of that diagram, exactly as
it already does not cascade to element/element_info. A full updateDB()
rebuild clears them, and the future wiring-list view INNER JOINs from
conductor, so orphan terminal rows never surface.

sources/diagram.cpp

@@ -1674,6 +1674,7 @@ void Diagram::addItem(QGraphicsItem *item)
 			conductor->terminal1->addConductor(conductor);
 			conductor->terminal2->addConductor(conductor);
 			conductor->calculateTextItemPosition();
+			m_project->dataBase()->addConductor(conductor);
 			break;
 		}
 		default: {break;}
@@ -1704,6 +1705,7 @@ void Diagram::removeItem(QGraphicsItem *item)
 			Conductor *conductor = static_cast<Conductor *>(item);
 			conductor->terminal1->removeConductor(conductor);
 			conductor->terminal2->removeConductor(conductor);
+			m_project->dataBase()->removeConductor(conductor);
 			break;
 		}
 		default: {break;}