rename `BillingPeriod` to `ProcessedBill` #399

Job	Run time
Rules Engine - Lint (3.11.3)	23s
⬣ Heat-Stack - ESLint	58s
Rules Engine - Build (3.11.3)	32s
Rules Engine - Mypy (3.11.3)	30s
Rules Engine - Pytest (3.11.3)	28s
ʦ Heat-Stack - TypeScript	49s
⚡ Heat-Stack - Vitest pyodide.test.ts	1m 8s
🚀 Heat-Stack - Deploy	0s
	4m 48s

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