Constraints

Technical

Free-tier hosting. Koyeb and Supabase free tiers rule out a staging environment, a message broker, and anything requiring always-on background infrastructure. These are accepted trade-offs, not oversights - restraint decisions reference this constraint (ADR 007, ADR 010).
LTS-only runtime. Java 21 and Spring Boot 4.x; major upgrades run as their own isolated change, never combined with other work.
Schema immutability. Applied Flyway migrations are never edited; every schema concern is a new migration.

Solo development with review discipline. No direct pushes to main: every change is a pull request against a protected branch with a required test gate, reviewed before squash-merging - main’s history reads as a sequence of complete, explained changes.
Portfolio purpose. The system targets technical reviewers in the German market; domain conventions (every sale invoiced, German document vocabulary) are requirements, not flavor.