feat: enforce gathering invoice uniqueness

[turip]

Overview

Let's mandate on a db schema level that there can be only one gathering invoice per customer per currency. The code have already enforced it, but let's have this constraint in the db too. (validated all of our deployments that this is true)

Summary by CodeRabbit

• Bug Fixes
  • Added database constraint to prevent duplicate gathering invoices for the same customer and currency, enhancing data integrity.

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[coderabbitai]

📝 Walkthrough

Walkthrough

Adds a partial unique index on the BillingInvoice table to enforce database-level uniqueness of gathering invoices per customer and currency. The change includes the schema definition and corresponding up/down migration scripts.

Changes

Cohort / File(s)	Summary
Schema Definition openmeter/ent/schema/billing.go	Adds unique index on (namespace, customer_id, currency) with conditions for non-deleted rows where status = 'gathering'
Database Migrations tools/migrate/migrations/20260113121259_gathering-invoice-unique.up.sql, tools/migrate/migrations/20260113121259_gathering-invoice-unique.down.sql	Up migration creates the partial unique index; down migration drops it

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• openmeterio/openmeter#3512: Also modifies BillingInvoice indexes in the schema, adding timestamp-related non-unique indexes alongside these gathering invoice uniqueness constraints.

Suggested reviewers

• chrisgacsal
• hekike

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'enforce gathering invoice uniqueness' clearly and concisely describes the main change: adding a database constraint to ensure only one gathering invoice exists per customer per currency.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat/enforce-gathering-invoice-uniqueness

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