feat: add TCO fields migration (refs #15)

Add database columns for Total Cost of Ownership:
- purchase_price, purchase_date
- insurance_cost, insurance_interval
- registration_cost, registration_interval
- tco_enabled toggle

Includes CHECK constraints for interval values and non-negative costs.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/src/features/vehicles/migrations/006_add_tco_fields.sql

@@ -0,0 +1,33 @@
+-- Migration: Add TCO (Total Cost of Ownership) fields to vehicles table
+-- Issue: #15
+
+ALTER TABLE vehicles
+  ADD COLUMN IF NOT EXISTS purchase_price DECIMAL(12,2),
+  ADD COLUMN IF NOT EXISTS purchase_date DATE,
+  ADD COLUMN IF NOT EXISTS insurance_cost DECIMAL(10,2),
+  ADD COLUMN IF NOT EXISTS insurance_interval VARCHAR(20),
+  ADD COLUMN IF NOT EXISTS registration_cost DECIMAL(10,2),
+  ADD COLUMN IF NOT EXISTS registration_interval VARCHAR(20),
+  ADD COLUMN IF NOT EXISTS tco_enabled BOOLEAN DEFAULT false;
+
+-- Add CHECK constraints to enforce valid interval values
+ALTER TABLE vehicles
+  ADD CONSTRAINT chk_insurance_interval
+    CHECK (insurance_interval IS NULL OR insurance_interval IN ('monthly', 'semi_annual', 'annual'));
+
+ALTER TABLE vehicles
+  ADD CONSTRAINT chk_registration_interval
+    CHECK (registration_interval IS NULL OR registration_interval IN ('monthly', 'semi_annual', 'annual'));
+
+-- Add CHECK constraints for non-negative costs
+ALTER TABLE vehicles
+  ADD CONSTRAINT chk_purchase_price_non_negative
+    CHECK (purchase_price IS NULL OR purchase_price >= 0);
+
+ALTER TABLE vehicles
+  ADD CONSTRAINT chk_insurance_cost_non_negative
+    CHECK (insurance_cost IS NULL OR insurance_cost >= 0);
+
+ALTER TABLE vehicles
+  ADD CONSTRAINT chk_registration_cost_non_negative
+    CHECK (registration_cost IS NULL OR registration_cost >= 0);