Fix 3+ bugs: email validation, error handling, exception classes

[Xenon010101]

Summary

• Fix [BUG] Add stricter email validation before SMTP verification #84: Add stricter RFC-compliant email regex validation in verifier.py before SMTP verification
• Fix exception handling: Replace nonexistent asyncpg.DuplicateTableError with asyncpg.DuplicateObjectError in storage.py
• Fix dead exception handler: Remove string.Formatter().format dead catch clause and replace with ValueError in discovery.py::construct_email
• Remove unused string import from discovery.py

[vercel]

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