Add proposed cable-sizing module

Isolated module (mirrors src/external-model/ dependency direction) that
adds an on-demand, DIN VDE 0298-4 referenced cable cross-section
calculator: a click-on-a-circuit input mask for laying method,
insulation, ambient temperature, grouping and voltage-drop limit that
suggests a cross-section for the circuit cableCrossSection/cableLength
fields. Applying a suggestion goes through the existing circuit.update
command (expectedRevision, undo/redo) unchanged - nothing here writes
to circuits directly or touches the revision/command system. See
docs/cable-sizing-module.md for the full rationale, API contract,
verification status and maintenance plan. Proposal, not yet reviewed
by the project owner - see the docs file.

423 existing + 11 new tests pass, build:api/build:web/typecheck:scripts
clean.

src/db/repositories/cable-sizing-calculation.repository.ts

@@ -0,0 +1,37 @@
+import { desc, eq } from "drizzle-orm";
+import type { AppDatabase } from "../database-context.js";
+import {
+  cableSizingCalculations,
+  type CableSizingCalculation,
+  type NewCableSizingCalculation,
+} from "../schema/cable-sizing-calculations.js";
+
+// Plain repository, no revision/command semantics - mirrors
+// application-repositories.ts's flat repository pattern, not the
+// project-command-transaction boundary. A calculation is an audit record,
+// not a project mutation.
+export class CableSizingCalculationRepository {
+  constructor(private readonly db: AppDatabase) {}
+
+  create(input: NewCableSizingCalculation): CableSizingCalculation {
+    return this.db.insert(cableSizingCalculations).values(input).returning().get();
+  }
+
+  listByCircuit(circuitId: string): CableSizingCalculation[] {
+    return this.db
+      .select()
+      .from(cableSizingCalculations)
+      .where(eq(cableSizingCalculations.circuitId, circuitId))
+      .orderBy(desc(cableSizingCalculations.createdAt))
+      .all();
+  }
+
+  markApplied(id: string): CableSizingCalculation | undefined {
+    return this.db
+      .update(cableSizingCalculations)
+      .set({ appliedToCircuit: 1 })
+      .where(eq(cableSizingCalculations.id, id))
+      .returning()
+      .get();
+  }
+}