fix: add schema migration for ownership_costs table (refs #29)

The ownership_costs table was created with an outdated schema that had
different column names (start_date/end_date vs period_start/period_end)
and was missing the notes column. This migration aligns the database
schema with the current code expectations.

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

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

backend/src/features/ownership-costs/migrations/003_alter_ownership_costs_schema.sql

@@ -0,0 +1,33 @@
+-- Migration: Alter ownership_costs table to match updated schema
+-- Issue: #29
+-- Description: Fix schema mismatch - rename columns, add notes, update constraints
+
+-- Rename columns to match code expectations
+ALTER TABLE ownership_costs RENAME COLUMN start_date TO period_start;
+ALTER TABLE ownership_costs RENAME COLUMN end_date TO period_end;
+
+-- Make period_start nullable (was NOT NULL as start_date)
+ALTER TABLE ownership_costs ALTER COLUMN period_start DROP NOT NULL;
+
+-- Drop interval column (no longer used in new schema)
+ALTER TABLE ownership_costs DROP COLUMN IF EXISTS interval;
+
+-- Add notes column
+ALTER TABLE ownership_costs ADD COLUMN IF NOT EXISTS notes TEXT;
+
+-- Update description column type to match new schema
+ALTER TABLE ownership_costs ALTER COLUMN description TYPE VARCHAR(200);
+
+-- Drop old check constraints
+ALTER TABLE ownership_costs DROP CONSTRAINT IF EXISTS chk_ownership_costs_type;
+ALTER TABLE ownership_costs DROP CONSTRAINT IF EXISTS chk_ownership_costs_amount_non_negative;
+ALTER TABLE ownership_costs DROP CONSTRAINT IF EXISTS chk_ownership_costs_date_range;
+ALTER TABLE ownership_costs DROP CONSTRAINT IF EXISTS chk_ownership_costs_interval;
+
+-- Update cost_type column to VARCHAR(32) and add new constraint with additional types
+ALTER TABLE ownership_costs ALTER COLUMN cost_type TYPE VARCHAR(32);
+ALTER TABLE ownership_costs ADD CONSTRAINT ownership_costs_cost_type_check
+  CHECK (cost_type IN ('insurance', 'registration', 'tax', 'inspection', 'parking', 'other'));
+
+-- Add amount constraint (amount > 0)
+ALTER TABLE ownership_costs ADD CONSTRAINT ownership_costs_amount_check CHECK (amount > 0);