[MAI-012] feat: Add DDL for contrato_addendas table

- Add contrato_addendas table for contract addendums/modifications
- Includes workflow status, amount changes, date extensions

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

schemas/01-construction-schema-ddl.sql

@@ -708,6 +708,33 @@ CREATE TABLE IF NOT EXISTS construction.contrato_partidas (
     deleted_by UUID REFERENCES auth.users(id)
 );
 
+-- Tabla: contrato_addendas (addendas/modificaciones a contratos)
+CREATE TABLE IF NOT EXISTS construction.contrato_addendas (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    tenant_id UUID NOT NULL REFERENCES auth.tenants(id) ON DELETE CASCADE,
+    contrato_id UUID NOT NULL REFERENCES construction.contratos(id) ON DELETE CASCADE,
+    addendum_number VARCHAR(50) NOT NULL,
+    addendum_type VARCHAR(30) NOT NULL,
+    title VARCHAR(255) NOT NULL,
+    description TEXT NOT NULL,
+    effective_date DATE NOT NULL,
+    new_end_date DATE,
+    amount_change DECIMAL(16,2) DEFAULT 0,
+    new_contract_amount DECIMAL(16,2),
+    scope_changes TEXT,
+    status VARCHAR(20) NOT NULL DEFAULT 'draft',
+    approved_at TIMESTAMPTZ,
+    approved_by UUID REFERENCES auth.users(id),
+    rejection_reason TEXT,
+    document_url VARCHAR(500),
+    notes TEXT,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    created_by UUID REFERENCES auth.users(id),
+    updated_at TIMESTAMPTZ,
+    updated_by UUID REFERENCES auth.users(id),
+    CONSTRAINT uq_contrato_addendas_number_tenant UNIQUE (tenant_id, addendum_number)
+);
+
 -- ============================================================================
 -- INDICES
 -- ============================================================================