Billable rates (1/3): data model + validation

[devin-ai-integration]

Summary

Bottom layer of a 3-PR stack adding billable rates and billed amounts to client reports. This PR only introduces the storage and validation primitives — nothing reads or writes them yet, so behavior is unchanged.

• clients.billing_rate DECIMAL(10,2) NOT NULL DEFAULT 0 and work_entries.billable INTEGER NOT NULL DEFAULT 1 in backend/src/database/init.js. The defaults are what make this layer non-breaking: existing rows/inserts get rate 0 (amount 0) and are billable, which is the behavior PR 2 will compute against. The DB is in-memory and rebuilt per process, so there is no migration.
• billingRate on the Joi client schemas: clientSchema defaults it to 0, updateClientSchema deliberately does not default it — a default on a partial-update schema would re-write the rate to 0 on every PUT that omits it.
• .strict() on the rate rules: without it this Joi version coerces 12.345 to 12.35 instead of rejecting it, so precision(2) alone would silently round client input.
• billable is intentionally absent from the work-entry schemas: the API does not accept it yet, and the column default covers every insert.

Stack: PR 1 (this) → PR #933 (service/API: billingRate in clients CRUD, totalAmount in reports, amount column in CSV/PDF) → PR #934 (UI).

Testing

cd backend && npm test (171 tests) with coverage above the backend/jest.config.js thresholds. New cases cover the rate default, the rejections (negative, over max, non-numeric, >2 decimals), the no-default-on-update behavior, and the two column definitions in the CREATE TABLE statements.

Link to Devin session: https://partner-workshops.devinenterprise.com/sessions/af51f484897545a8bc1df69511f47ed4
Open in Devin Desktop: https://partner-workshops.devinenterprise.com/desktop/session/af51f484897545a8bc1df69511f47ed4?variant=devin
Requested by: @sumitshatwara

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
2.5% Duplication on New Code

See analysis details on SonarQube Cloud