setup: wire golang-migrate runner into server startup

[Lewiskunta]

Implements: database migration runner.

Changes:

• Installed golang-migrate/v4, pgx/v5
• Replaced SELECT 1 placeholder with real migration 000001
  enabling the pgcrypto extension for UUID generation
• Created internal/db/migrate.go with RunMigrations()
• RunMigrations() is idempotent - safe to call on every startup
• Handles dirty state detection and logs migration version
• Wired RunMigrations() into cmd/server/main.go startup sequence
• Server exits with a non-zero code if migrations fail
• Unit test for invalid database URL handling

Verified against live Docker Compose postgres:

• Migration 000001 applies on first run
• No re-application on subsequent runs (ErrNoChange)
• schema_migrations table created by golang-migrate
• pgcrypto extension enabled in database