rename `BillingPeriod` to `ProcessedBill` (#277) #29

Job	Run time
Rules Engine - Lint (3.11.3)	24s
Rules Engine - Mypy (3.11.3)	27s
Rules Engine - Build (3.11.3)	29s
⬣ Heat-Stack - ESLint	59s
Rules Engine - Pytest (3.11.3)	23s
ʦ Heat-Stack - TypeScript	56s
⚡ Heat-Stack - Vitest pyodide.test.ts	1m 5s
🚀 Heat-Stack - Deploy	14s
	4m 57s

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