add email auth validation with middleware

[umputun]

PR adds a middleware checking "user" query param similarly to the current UI implementation, with ^[\p{L}\d\s_]{4,64}$ regex.

This is needed to prevent direct API calls from sending non-sanitized data into /auth/email/login endpoint. Also, see related PR in auth lib

[coveralls]

Pull Request Test Coverage Report for Build 2440411171

• 32 of 32 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.07%) to 84.056%

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Totals
Change from base Build 2429336973:	0.07%
Covered Lines:	5741
Relevant Lines:	6830

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💛 - Coveralls