Reduce duplicated shared business logic

[dykdee]

Closes #602

Summary:

• moved repeated subscription spend math into @syncro/shared
• moved shared URL/email/masking helpers into @syncro/shared
• rewired client and backend callers to use shared helpers
• added tests and docs for shared business logic

Tests:

• npm run lint:deps -w client
• npm run typecheck -w shared
• npm run build -w shared
• npm test -w backend -- shared-business-logic.test.ts
• npm test -w backend -- telegram-command-service.test.ts
• npm test -w backend -- validation.test.ts
• npm test -w client -- lib/tests/validation.test.ts

[drips-wave]

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