fix(migrations): make V5/V6 idempotent with IF NOT EXISTS

Avoids Flyway errors when columns already exist in the DB due to
migration history mismatches from parallel feature branches.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/resources/db/migration/V5__add_notes_to_persons.sql

@@ -1 +1 @@
-ALTER TABLE persons ADD COLUMN notes TEXT;
+ALTER TABLE persons ADD COLUMN IF NOT EXISTS notes TEXT;

backend/src/main/resources/db/migration/V6__add_birth_death_years_to_persons.sql

@@ -1,2 +1,2 @@
-ALTER TABLE persons ADD COLUMN birth_year INTEGER;
-ALTER TABLE persons ADD COLUMN death_year INTEGER;
+ALTER TABLE persons ADD COLUMN IF NOT EXISTS birth_year INTEGER;
+ALTER TABLE persons ADD COLUMN IF NOT EXISTS death_year INTEGER;