Add unique constraint to prevent duplicate measurement data

[maltehuebner]

Summary

• Add unique constraint on (station_id, pollutant, date_time) to data table
• Handle UniqueConstraintViolationException in PostgisPersister
• Also adds null check for ValueDataConverter::convert() return value

Context

Without a unique constraint, repeated imports (e.g. retries, overlapping fetches) could insert duplicate rows for the same measurement. The constraint ensures data integrity at the database level.

Test plan

• Generate and run Doctrine migration: php bin/console doctrine:migrations:diff then php bin/console doctrine:migrations:migrate
• Verify importing the same data twice does not cause errors
• Verify normal import flow still works

🤖 Generated with Claude Code