[IP-268]: bill project tasks to a draft invoice

[nielsdrost7]

Addresses #268 (kept open per workflow — PRs stay draft).

What

• ProjectBillingService::billTasks(Project, array $taskIds): Invoice — creates one invoice item per selected task (item name = task name, quantity 1, price = task_price, task's tax rate, task_id link). Appends to the client's existing draft invoice when one exists, otherwise creates a new draft with a null invoice number (numbering is assigned when the invoice leaves draft — consistent with the duplicate-prevention observer on PR [IP-513]: duplicate-number prevention via Observers #540, which allows multiple nulls for drafts).
• Only completed tasks that are not already on an invoice (invoice_items.task_id) qualify — double billing is impossible.
• Bill Tasks row action on the projects list (gated by create-invoices) opens a checkbox list of billable tasks; guard failures surface as a notification.
• Invoice totals are refreshed from the item subtotals after billing.

Deviation from the SMART plan: no redirect to EditInvoice — that page only exists on PR #544's branch, not on develop yet. Trivial to add once #544 lands.

Tests

ProjectBillingTest — 5 tests: new draft creation, append-to-existing-draft, double-billing rejection, non-completed rejection, and modal option filtering.

Note: upstream CI fails at job startup on every branch (no runner assigned) — validated locally; the 24 pre-existing Projects failures (permission-seeding 403s) are unchanged.

🤖 Generated with Claude Code

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