feat: add after-user-created hook

[cstockton]

This PR implements the after-user-created hook which runs whenever a new user has been successfully created.

Summary:

• Add triggerAfterUserCreated method to the *API object in internal/api/hooks.go
• Update user creation paths to call triggerAfterUserCreated after a new user is persisted:
  • internal/api/anonymous.go
  • internal/api/external.go
  • internal/api/invite.go
  • internal/api/mail.go
  • internal/api/signup.go
  • internal/api/samlacs.go
  • internal/api/token_oidc.go
  • internal/api/web3.go
• Extend createAccountFromExternalIdentity to return an AccountLinkingDecision to detect newly created accounts
• Add full end-to-end verification of the new hook in internal/api/e2e_test.go

[coveralls]

Pull Request Test Coverage Report for Build 17960008964

Details

• 49 of 102 (48.04%) changed or added relevant lines in 10 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.04%) to 67.658%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/api/anonymous.go	1	3	33.33%
internal/api/hooks.go	8	10	80.0%
internal/api/invite.go	1	3	33.33%
internal/api/mail.go	7	9	77.78%
internal/api/signup.go	1	3	33.33%
internal/api/web3.go	16	20	80.0%
internal/api/token_oidc.go	0	9	0.0%
internal/api/samlacs.go	0	10	0.0%
internal/api/external.go	10	30	33.33%

Files with Coverage Reduction	New Missed Lines	%
internal/api/mail.go	1	65.25%

Totals
Change from base Build 17947966352:	-0.04%
Covered Lines:	13035
Relevant Lines:	19266

---

💛 - Coveralls