feat: support email invitation for unregistered users

[boris-w]

Use the user's password and account to determine whether to register or not.
Registered: Password has been set or account already exists (created by social login).
Unregistered: No user exists, or no password exists and no account exists.

[coveralls]

Pull Request Test Coverage Report for Build 9314269282

Details

• 45 of 68 (66.18%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+63.7%) to 82.509%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/nestjs-backend/src/features/auth/strategies/github.strategy.ts	0	3	0.0%
apps/nestjs-backend/src/features/auth/strategies/google.strategy.ts	0	3	0.0%
apps/nestjs-backend/src/features/invitation/invitation.service.ts	16	19	84.21%
apps/nestjs-backend/src/features/auth/auth.service.ts	6	20	30.0%

Totals
Change from base Build 9301169849:	63.7%
Covered Lines:	26053
Relevant Lines:	31576

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💛 - Coveralls